I've only watched the first 26 episodes of Part 3 (and bits here and there for the rest), so I cannot fully judge but it looks like it's going to be my second favourite series after Part 2.
1. Part 3 got a bad reputation for years due to so many factors. The Japanese had issues watching the show back in the day so not many got to watch it compared to Part 2 or even reruns of Part 1, still did better than expected though. Enough to get 50 episodes but not enough for the planned 52. It had bad scheduling as the network favoured baseball so episodes were hit and miss in terms of airing, getting to the point where episodes were completed months before air, a rarity in anime. The final episode for the ones who were fans was only aired at selected regions at Christmas Day and had to wait until a VHS/Laserdisc release to watch it. It was why NTV became the network for Lupin since. It's still the lesser known series as the nostalgia factor for Part 1, Mamo, Cagliostro and Part 2 to a lesser extent is really high.
The Italians had a really censored dub that took decades to get a complete dub, episode 13 in particular was just over half an episode left with what got cut out. Due to the cuts, ironically it was seen as the weakest and "childish" series. By comparison Parts 1 and 2 were less censored.
In terms of English speaking fans, originally very few got to watch Part 3. The original fan translations were really bad and didn't make sense. The "Crunchyroll" translation was miles better but still has some issues. It also didn't help that it was compared to The Legend of the Gold of Babylon due to sharing an animation style of the latter episodes. It had a stigma where it was "this cartoony series" when it's really not that cartoony outside of its appearance and fuelled by the Pink Jacket episode of Part 5 where honestly... it's more like the wacky part of Part 2. Only a few episodes got strange, one was written by Seijun Suzuki and one with a dolphin but a majority were more grounded than some of Part 2 focusing on the US in the 1980s or Middle Eastern/Cold war like settings. There were strange moments, questionable even (Episode 3 especially) but nothing like cat eating pencil shavings, Jesus had a sister who was the villain and a vampire, Lupin becomes Superman (twice!), popcorn powered rocket or Lupin talking to a ghost of Part 2 (the Part 3 take with Goemon was more logical). Some are even deeper than what you expect at the surface like Lupin hanging around a poorly girl or Lupin being a bartender helping a woman, the latter was both a manga chapter and an anime episode as it was written by the same person. The reputation for Part 3 has grown especially since the past decade onwards and is no longer considered the worst mainline series. That title usually goes to Part 6.
For the series itself, it's the inconsistent animation where it can change style in an episode itself putting off people (Part 2 has a bit of this too but is more consistent in style, this series was more animator lead). It can go off model and it does have a lack of polish from time to time but does have a charm. There were three main character design styles with the cartoony one being at the latter episodes. Personally I'm not keen on Lupin's design in the mid episodes where he looked like a "camel" with his long cleft chin and call it the "worst" Lupin design despite liking the early and latter episodes but I am really fond of Jigen's design in the early episodes. Fujiko also suffered from at least four different designs, none are really appealing. Some of it was down to 1980s fashion though.
Plot wise. I've touched on the perception but because it doesn't have those series highs (very few of Part 3 episodes ever make a fav Lupin episode list for the majority*), it is seen as more flat. The US setting, in particular New York or the war setting were seen as repetitive. There were issues with Fujiko I believe, especially in the early episodes with the villains. Lupin didn't feel like he was in too much danger (also an issue with some more modern entries) and for the majority of the series fun over emotions that can put people off if they expect drama. Sure there are emotional moments but nothing really impactful or happens too late. There is also a common theme of betrayal usually of an old friend, again repetitive especially early on. Compared to Parts 1 and 2, not as much interaction between the main characters and sometimes feel that they're in the background that extends to the TV Specials, due to one of the writers for Part 3 wrote a fair amount of them (Voyage to Danger is a remake of the final episode). Like one episode Goemon appears just for a couple of scenes without anything to say. Yet sometimes the interaction does have some golden moments. A couple of episodes were really faithful to the manga and the manga does get disjointed in terms of plot, again Parts 1 and 2 do have this too (more so on much of Part 1 where you had a faithful manga scene and then something lighthearted because Miyazaki and Takahata put that in throwing out what Osumi had worked on).
*- In terms of polls, episodes 30 (A Cocktail Named Revenge), 37 (Pops Gets Very Angry), 49 (The Day When Pops Gets Adopted) are the episodes that usually get mentioned. The episode with Goemon and Seiryu (24, Sleep Deeply My Friend) also gets mentioned.
Another subject and one that is more recently noted is that it also suffers from stereotyping due to the character design of the black characters that makes it poorly aged (even the very few times that Part 2 had a black character didn't have this problem). It's more of an issue in The Legend of Gold of Babylon but is still there in a couple of episodes, I think Manhattan Crisis was one of them.
One major thing is that the music considered to be one of the weaker entries, theme song Sexy Adventure aside. It was when Yuji Ohno went more of an electronic take and again early on relies on the same short jingles/tracks and Sexy Adventure remixes, by the end reuses Babylon music. It doesn't have the same impact of Parts 1 and 2 or the later series that remixes the Part 2 tracks. It does have a few good tracks though.
2. YES! The OP mentioned of preferring comedy and fun, well... Part 3 does have that. It does have some fun episodes especially by the second half. The animation can get really good and fluid, even the earlier more consistent style still has some nice looking shots. You can tell that the animators probably liked working on the show and unlike Part 2 (and Parts 4, 5 and 6 for that matter due to different times), there weren't any what are considered cheap, stiff looking and rushed out. A couple of off model episodes yeah but that's more down to the animator and still is one of the closest anime towards the Golden Age animator mentality of individuality (or reanimated videos on YouTube).
Usually when it comes to trying a few episodes, it's hard with Part 3 because either you choose the best and ignore the rest or have a bad first impression (my first episode was actually Episode 11, so restarted from Episode 1 and a rewatch of 11 held up better). Just go with the flow and 50 episodes isn't too much to risk. If you don't like it, you don't like it.
All I can say... When you get to Episode 49, you'll be in for a treat.