The treble is domestic league, domestic cup and continental trophy (think Man U's in '99).
Because of the overlapping schedule of MLS and CCL, for us, the next available one would be:
2009-10 CONCACAF Champions League (April)
2010 Voyageurs Cup (July)
2010 MLS Cup (November)
Optional quadruple: 2010 World Club Cup (December)
Imagine culminating the perfect calendar year with a World Club Cup win in December?
That would also effectively mean two straight Voyageurs Cups.
We could win a treble with any three tournaments, including Supporters' Shield and Superliga (equivalent of the UEFA Cup now that teams can't be in both CCL and Superliga).
In that same light, the double would be MLS Cup and Voyageurs Cup.
A double would be any two tournaments, arguably including the Trillum Cup and the Pan-Pacific Championship.