There are three players: A, B and
A has $25, B has $50, and C has $100
A goes all-in
B and C call
Later in the hand, C makes B fold
Now, as B folded, will B be allowed to play for the main pot?
No, B folded. They are no longer in the hand. Cards are tabled, main pot of 75$ goes to A or C, depending on who has the best cards. If a side pot exists it goes to C.