Is there a way to get rid of the 3, and the sideways square bracket under it, that appears above a stave where there is a triple?

I'm trying to enter something in 12/8 time, using 4/4 time with each quarter note divided into 3 divisions (as in swing time).

Showing a triple over what is supposed to be 3 8th notes doesn't seem right.

This might work:
You can change the beat resolution on any beat. In editable notation mode, change that beat to 4/4 as shown:

OK, that works. But it changes the timing, I think. It sounds the same to me, but in the notation the middle note appears to be held twice as long as the outside notes.

You can adjust both the length and position of individual notes. In editable Notation mode, right-click on the note-head to change it. That may help to give you the result you require.

OK. Since there are triples in every bar, I have decided just to leave them in. The time signature says 4/4 in BiaB. The original time signature was 12/8, but it sounds correct.

I understand that BiaB treats Swing styles as triplets. I think this is why you are getting the results you see.