A multiway pot is a pot contested by three or more players. Every extra player makes it more likely that somebody holds a real hand, so bluffs work less often, value hands need to be stronger, and heads-up rules like MDF stop applying.
If each opponent folds to your c-bet half the time, both fold only about a quarter of the time, so a bet that was profitable heads-up with air now loses money. The adjustment is simple: check your air, even in position on boards you would always c-bet heads-up, and bet the hands that are happy to be called — top pair with a good kicker, overpairs, two pair, sets. Size can stay normal; what changes is which hands bet.
Position is worth even more multiway, because you see every player act before you decide. Out of position, check far more, including hands you would bet first heads-up. When someone bets into two players, or raises with a third still to act, his range is much closer to two pair, sets and big draws than the same action heads-up — so fold more one-pair hands, and do not defend to MDF: the defense is shared among the players left in the pot. Strong draws gain value by calling, because the players behind can come along and pay you when you hit.
Worked example
FAQ
Should you c-bet in multiway pots?
Much less often with air. A bluff has to get through every player, so check your missed hands and bet the ones that want to be called: top pair with a good kicker, overpairs, two pair and better.
Does MDF apply in multiway pots?
No. Minimum defense frequency is a heads-up idea. When the bettor faces two players, the defense is shared between them, so each player can fold far more than MDF suggests.