Some context would help figure out what would work best. I suspect the answer is not a switch but something else.
The easiest switch would look like this from the audience's perspective. You take three envelopes out of a stack of envelopes of envelopes bound by a rubber band. People put things in the envelopes. You collect the envelopes and put them on top of the stack of envelopes as you move across the stage. You then take the envelopes on the top of the stack and hand them to another spectator.