i've double hooked a few fish in my time but never treble hooked before. if two trout are close together and spot the flies at the same time, my guess is they take a first in gets fed approach, and go for the nearest fly to them.
hooking them is one thing, netting them is the difficult part. if there's somebody close by, i usually ask them to lend a hand (and their net). this stops the first fish getting overstressed whilst trying to net the second.
by the way, real mackrel will readily take a fly as well