First as someone who runs 3Gun matches, always push the limits of your core group of shooters skills with a few very difficult shots, but not overly hard that they get discouraged and don't come back.
But
Can't hit a 16x20 at 100yds ? ? WOW !
Are these brand new shooters? if so that's barely acceptable.
Flashers are the easiest way to see hit out to about 350 yds, we use strobes from 400-600 yds.
Last month we had targets that size, offhand at 200yds after running 200 yards, then flashers out to 350 off a roof top.
It was an EXTREME stage and about 75% of they guys timed out,5 minute par time.
but most said they wanted to do it again.
The temp got to about everybody, 96* and 96% humidity, it was the hottest day of the year so far. Just breathing felt like you were drowning.