If I am running a formtaion with two split backs and a full back in the center, then are all the runs up the middle going to go to the fullback?

Basically it works like this...

Running Left-
Use first to be present:
HB(L), FB(L), HB(C), FB(C), HB(R), FB(R)

Running Right-
Use first to be present:
HB(R), FB(R), HB(C), FB(C), HB(L), FB(L)

Running Centre-
Use first to be present:
HB(C), FB(C), HB(L) OR* HB(R), FB(L) OR* FB(R)

*randomly determine for each play if both present