If ALL THREE of them are able-bodied, sure-footed and relatively
fearless about heights.

If by "average pitch" you mean at or less than 6/12. Otherwise, they
just wind up being too tall and top-heavy. It can still be done, but
requires more guys and more, bigger ladders.

Guy A stands on top plate while B and C carry truss over and hand the
end up to A.

B climbs ladder and stands on top plate. Meanwhile A has shimmied out
onto an already-installed truss.

C pushes from the ground, B pulls from the top plate and A walks the
end of the truss to the other wall.

C climbs ladder while B walks out into the trusses, all the while
holding the truss steady.

A and C nail to the top plate, B braces with truss locks or scrap 2x
lumber. Remember to use duplex nails for this. DAMHIKT.

Setting the first truss (not the gable end) goes the same way, except
there's obviously no truss for A to walk across. He has to climb down
a ladder, gorilla-fist the truss across the building, and back up the
ladder to the opposite top plate.

Keep in mind, this is only my experience. As always, Your Mileage May
Vary.

At any rate, I hope it helps.

-Phil Crow