Changing key signature two thirds of the way through a set of triplet notes

Hi, I am putting the Bernstein / Sondheim song 'Maria' into Musescore. The song changes key two thirds of the way through a set of triplets! i.e. Two thirds of the way through the last beat of the bar. So there are two notes of the triplets in the original key (C major) and the last one in the new key (E major). I know how to enter triplets and I know how to change key signature. I just don't know how to change key signature after the first two notes of a set of triplets and before the third one! Any help much appreciated! The issue arises in the treble clef for actual notes and is mirrored in the bass clef with rests similarly split across the key change.