As others have, I find it hard to limit to three, but I'll try, with the caveat that of course Doug is right about the power of the internet and the generosity of strangers in sharing their ideas.
(1) Golite shoes: not a favorite for everyone, but the wide toe box allows me to backpack (at least, without foot surgery)
(2) Andrew Skurka's video and related instructions for making a dead simple, light and effective cat food can alcohol stove; a fairly recent discovery for me, so maybe it won't hold up in my top 3 long term, but it's great to have something this effective that I can recommend to folks that had previously never heard of alcohol stoves.
(3) For #3 I guess I'll go with the whole freezer bag cooking thing too; I sure enjoy not having to clean pots, and the related process of easy-at-home, easy-on-trail, and lower price.
After I've done some trips with it, my new Gatewood Cape might well make it into the top 3; it seems like pretty genius design to me.
Then the usual praise for tarptent.com products, GG packs, aqua mira, Ursacks, Montbell thermawrap and sleeping pads, C.F. trekking poles, super-absorbant pack towels, ...