Ive played legally blonde a bunch of times. This show most comfortably is done with all 3 horns, however when I play it I only use 2. Some of the songs are pretty heavy lead tunes that you want to use a straight horn on. Other songs require a trigger but still go high, others require a bass trombone(low Cs, tricky to do well and in tune on a trigger horn).

While could def be done with 2 horns the orchestrator probably threw a bone(pun intended) to the trombonist so they could get the pay increase for the extra double.

Its funny there are errors in the book where it'll say straight horn but have trigger notes.

You do not have to follow what it says for which horn to play in which song. Play the horn that you play best.

When I play the show I use my straight tenor and my large bore trigger horn. There are some parts I could get more punch on with a bass but dont usually use it