I have a 10' crank-out awning and started with 3 brackets (from
an earlier awning) to find that I kept bending/breaking brackets
and the gutter.  If you sight from back to front, you see the
van has a bow.  Crank-out awnings are stiff and don't like to
be bowed, so there is stress.  Two brackets work fine for me now.

I started with three because I formerly had a hand-roll-out awning,
which is more flexible--that is, the case is more flexible--and
three brackets worked fine with that for 15 years.

With a heavier crank-out, I like two beefy brackets (such as GoWesty)
with the bolt into the sheetmetal to keep the awning from trying
to flex the gutter.

Richard