For things to work out nicely then you will want the following relationship between the three unknowns a, b & c :
a = b+1 = c+1
For example consider a=8 (teams A to H), b=7 and c=7.
S1 S2 S3 S4 S5 S6 S7
R1 AB EF GH CD
R2 FH AC BD EG
R3 FG AD BC EH
R4 DH AE CG BF
R5 DG BE CH AF
R6 CE DF BH AG
R7 BG DE CF AH
Here is another example (http://mathworld.wolfram.com/RoomSquare.html) of the same thing.