The space you have allocated is insufficient for three fruit trees, even if you plant dwarf varieties. It would be best to allow 12 feet between the trees. Although you might be able to squeeze in two dwarf trees, the space is ideal for one. If other apples are growing in the neighborhood (including crab apples), sufficient pollen will probably be available to make the lone tree fruitful.
It's impractical to try to improve the soil in the entire yard. When you plant, remove the rock and dig a hole twice as wide as the root-ball and thoroughly mix up to one-third volume of organic matter (peat, compost, etc.) with the native soil when you fill the hole. Contact your local Extension office for more information on selecting, planting, and maintaining fruit trees in your area.
