If kids were on a state trip for a school-sponsored event, I think you should have worked around it and found a way for them to audition.
The girl who did a video audition, did she communicate ahead of time about it? Was that offer made to the other two?
I'm not 100% clear from your post, were the students physically gone for the state event during the auditions? Or could they have made it to the auditions, but chose not to because they had been busy during the week?
I'm not clear about the student who asked for 20 minutes, was this person part of the state event? Sounds like this person was just late/forgot and missed the audition. I had a student miss their drum major audition last week, that was enough to cut them from that position (they probably would not have made it regardless).
Does the guard sponsor work for you/the band? Or is the guard sponsor separate somehow? Will this person be around in the fall regardless?
If you trust that the girls did in fact tell the guard sponsor, then I would probably find a way to let them audition. I would also have a serious conversation with the guard sponsor about communication. Sounds like that would cause strife between you and the guard sponsor.