Is there anything like greater restoration available to me at level 3? Or is there something I can do as a level three sorcerer to undo the effects of a Intellect Devourer's ability score reduction on a fallen player character?
While you can't do anything personally, you may be able to go to a town large enough that has spellcasting services where you could employ someone that can cast Greater Restoration.
Whether or not this is an actual possibility will rely on your DM and the world you play in.
Plot Hook help
Getting this done is a great plot hook set-up! Whether it's in the search or in a barter for payment - there's a lot here for the group to do.