I'd grow things that are difficult or expensive to buy. Around here that would be Meyer lemons (some years I can find them in quantity some I can't - but they're finicky to grow), Black Mission Figs ($5/pint and only available 2 weeks or so), and Sour Cherries (available 1 day/year). Pomegranate or avocados would be my runners up.
Although a nut tree is also really nice to have. Pecan or walnut trees tend to get pretty big, but almond trees stay a decent size. Think they need cross-pollinization though.
Not trees, but they do take up plenty of space - cane berries are another thing to consider (blackberries, raspberries, boysenberries, marionberries, etc.).
Oh, I will say I wouldn't recommend planting multiple stone fruit trees (apricot, peach, plum, nectarine, etc.) if you're limited to three. If you get a good weather year, you'll spend ALL SUMMER canning, and won't be able to GIVE away the excess. My grandparents had at least one of each (plus plumcots, 3 cherries, figs, apples, nuts, grapes, berries, melons, etc.) and I remember my grandma spending the hottest parts of summer stuck in the kitchen canning/jamming. While I enjoyed climbing the trees and eating the products of the garden all year round, spending 110F days stuck in the kitchen with a water bath canner SUCKED.
Cristeen ~ Always remembering our warrior ~ Our is 3, how'd that happen?!?!
We welcomed another warrior in May 2012!!
2012 Decluttering challenge - 575/2012