Triple 10 or 13 should work OK. Use 1 pound per inch of tree diameter. Spread out to the drip line. There's a few on this site that have planted them. I have none in my area. Maybe they will catch this post and give you more specifics. Would suggest to go a little lighter if you just have 3X13. Old timers around here say that trees need a little iron in the soil. Old rusty nails placed in a little ditch will sork.

Russ