I went to order a gen3 timing belt cover today but, there are 3 part numbers.
the mr2 celica and caldin all have different part numbers for the lower 3sgte cover.
Does anyone know the differences? The top covers are all the same numbers. 
I've put a gen 3 sump and pump on my gen2 so i dont really know which one to buy. If it matters

Let me try shake off the rust, I looked into all this a while ago and had a look at all the covers first hand, but cant remember 100%.If you've got a gen 3 sump and pump use the gen 3 mr2 cover.  I believe the holes in the cover for the mount bracket differ between the GT4/Caldina and the MR2.    The caldina cover I believe is different around the bottom portion to allow for mounting of the CAS.  The caldina sump upper portion also differs from the gen 3 one.

thanks, i'll look out for the mr2 set up then, It's a bit expensive to buy new at the moment, hopefully someone wrecks an engine one day

3 part numbers may refer to top inside (no3), top outside (no 2), and bottom covers (no1).
Anyway, check out Toyodiy for the part nos.
On the gen2 3SGE the top outside timing covers are different, depending on whether you have you have the spring type tensioner or the hydraulic tensioner.
 
I think all the top timing covers are the same for all NA's and turbos with the hydraulic tensioner.