In a multiplayer game can you attack one opponent and redirect combat damage to another with an effect?

In a multiplayer game, when you declare attackers you must choose which opponent (or their planeswalker or battle) each creature is attacking (CR 506.2). Combat damage from that creature is then dealt to the chosen defending player or permanent — not to any other player.

There is no general rule that lets you freely redirect combat damage to a different opponent. Damage redirection only happens when a specific card effect explicitly says so. For example, some cards or abilities say 'the next N damage that would be dealt to [target] is dealt to [something else] instead' — only those effects can move damage away from the declared defender (CR 614.9).

Without such an effect, the damage simply goes to whoever or whatever was legally declared as the attack target. Choosing one opponent as your defending player and hoping the damage hits another is not possible under normal rules.

Example: You attack Player B with a 5/5 creature in a three-player game. Player A controls a Pariah enchanting one of their creatures. Unless Player A's Pariah specifically redirects damage that would be dealt to Player B, your creature's 5 combat damage goes to Player B as normal. Pariah only redirects damage dealt to its controller, not to another player.

Bottom line: declare your attack against the opponent you want to deal combat damage to. Redirection to a third party requires an explicit card effect that creates that redirection (CR 506.2, CR 614.9).