The TT does have a few variations on springs, the quattro sport had different springs, the s line was a differnet spring too, i think if you order 18 inc alloys it came with slightly lower suspension too so a different spring again. Theres nothing to stop you from using tt quattro springs on the S3 though and if you check on vagcat you'll probably find they share some of the springs listed.
A quick look on vag cat shows the TT and S3 share the following rear springs:
Part No.s and paint marks -
1J0511115CR 5 marks violet
1J0511115DD 2 marks silver
1J0511115DE 3 marks silver
8N0511115E 1 mark red - this is also noted as for sports type running gear under the TT which might be the sline or quattro suspension
The following was also listed for the S3:
1J0511115BM 5 marks blue
The following was also listed for the TT:
8N0511115F 2 marks red for sports type running gear
8N0511115G 3 marks red for sports type running gear
1J0511115DF 4 marks silver