Bringing this back because I now absolutely notice this. Season 3 has an extremely “at home” type of feeling, like we know all the characters and the establishing work that was done in seasons 1 and 2 is no longer needed. Here are factors I think make season 3 feel unique:
1. The very small pool of writers, especially in the second half (mostly these episodes are just Kent Osborne, Merriwether Williams, Paul Tibbit and sometimes C.H. Greenblatt, Mark o’Hare and Kaz. You can feel these people’s chemistry in the writing.
2. This was probably the time SB’s workplace was most personal, informal and “fun”, because 1) the small writer pool as I mentioned and 2) as far as they knew they were just wrapping up a highly successful cartoon.
3. In general season 3 is the exact opposite of season 6 or 7 as while sometimes the characters act jerky to teach a lesson most of the time all the characters even Squidward are relatively positive and working together. (On a side note, I think seasons 9B and 10 tried to re-capture this feeling, but it was hard because of how much it changed)
4. This season was the sweet spot between the extremely nautical themes of seasons 1 and 2 and where it just became like a sort of “wacky”, aesthetically all over the place cartoon in season 4. The colors are light and welcoming in pretty much every scene from this season and it seems like Bikini Bottom was painted more as a cozy small town compared to other seasons.