I made my first biscuited miter joint last week; with two 15" wide panels
and three biscuits.
It did not come out well; no matter how much I fussed with it I couldn't
eliminate the gap. Possibly no one would have noticed it, but I sure didn't
like it. (it couldn't have been too bad, as it held my full weight jumping
on it...)
So, I am redoing it, being more careful that my cuts are perfect, and only
using two biscuits.

Well, when I dry-fit it, there still a gap I can't get rid of; the gap is
uniform all across, so my cuts are good. I marked up a biscuit with a
central line, and the central line falls exactly on the edge of the slot. I
have thinking that maybe the clamping on 90degree joints is adequate to make
it work, but the clamping on 45degree joints won't push them together, and I
have to make my slots a hair deeper.

Does this make any sense, or is my problem likely to be elsewhere?

I am not thrilled about the recutting my slots, as they are likely to end up
a bit wider than I would like; maybe I can get away with just deepening one
side?