The trumpet part in Of Emerald Shires only goes up to a C. I don't know of any other grade 3 that is that friendly on trumpet range.

Look at the three Holsinger hymnsongs - may be a good fit.

If you're looking for something more up-tempo, Tricycle by Boysen is a great piece, and doesn't go too high - but one of those trumpet players has to play a very exposed solo.

You also have between 3 and 4 months to get trumpets up to the top of the staff. Have them work scale drills and sustain the upper note of scales to work on their range.