This is the 850th most frequent Japanese word.
決まっ
"Decided" or "settled."
決まった is used here to indicate that the result has been determined or finalized.
試合の結果が決まった。
The result of the match has been decided.
決まります is used here in its future tense to express that a decision will occur later.
そのスケジュールは明日までには決まります。
The schedule will be decided by tomorrow.
決まっている illustrates a state resulting from a prior decision.