Curate, connect, and discover
They don't really explore Fabian being the only member of the original 3 in S3 at all. In fact, his character is reduced to background role for most episodes save for ones where they explore his dynamic with KT (one of the only saving graces of S3). The problem with the season as a whole is the original cast as a whole is wasted either by being reduced to romance subplots (Joy and Jerome especially who no longer are in the mystery) or reduced to one trait (Jealous Patricia and nerdy Fabian).
you know what your so right for this, honestly wouldn't be surprised if the goal of adding new characters to season 3 ( who I assume are all either first years or something similar because adding a whole cast of new last years is wild to me) was to create a setting for a possible season 4. cuss like obviously its set in a school so the cast can't stay the same forever and also the main cast was mainly young actors who probably don't wanna be tied down to a teens show for their entire possible career and stuff so new characters who clearly have a connection to the wider working plot makes sense. but no yeah as you rightfully mention because of this ( possible and probably inaccurate) setup I mentioned it causes the cast who stayed to become less relevant. their main character arcs have already been told in some ways, and whilst there are obviously more ways to go with a character sometimes its best not to. and like this ends up with, as you said, them becoming revolved around relationship drama that at times can feel really out of character? and also touch off the plot. its actually really interesting to delve into how both fantastic and utter crap this season is, and how sometimes those reasons intersect. and like obviously all the seasons had MAJOR issues, I don't want anyone to think I'm just hating on season 3 for no reason or because it changed the flow I just mainly remember me disliking season 3 as a kid and therefore obviously tend to criticise it more publicly I guess