There are 9×8×7/6=84 combinations of 3 squares out of 9.
If the squares are labelled A, B, C, D, E, F, G, H, I the combinations are:
ABC, ABD, ABE, ABF, ABG, ABH, ABI, BCD, BCE, BCF, BCG, BCH,
BCI, CDE, CDF, CDG, CDH, CDI, DEF, DEG, DEH, DEI, EFG, EFH,
EFI, FGH, FGI, GHI, ACD, ACE, ACF, ACG, ACH, ACI, BDE, BDF,
BDG, BDH, BDI, CEF, CEG, CEH, CEI, DFG, DFH, DFI, EGH, EGI,
FHI, ADE, ADF, ADG, ADH, ADI, BEF, BEG, BEH, BEI, CFG, CFH,
CFI, DGH, DGI, EHI, AEF, AEG, AEH, AEI, BFG, BFH, BFI, CGH,
CGI, DHI, AFG, AFH, AFI, BGH, BGI, CHI, AGH, AGI, BHI, AHI.
These can be converted into shapes by arranging the letters into a 3 by 3 grid and using only those squares indicated by the letter patterns:
Some patterns above have the same shape; others contain disconnected squares.