I'm an amateur at trees, but definitely an aspiring expert. So your question interested me. One immediate thought comes to mind, could the lack of moisture nera the tree be caused by the tree canopy? The root ball would seem to be more likely but possibly not.
With the temperatures you describe and the value and commitment associated with those three trees, I would measure the moisture in the soil as near the root ball as possible in the early stages of growth. Next year, the roots will have spread, this year they are unlikely to have spread much.
Thanks for finally clearing that up. It has to be the differance of the media in the rootball compared to the surrounding soil since the trees are too small to have a canopy. I will use the reading I get near the rootball now, because with the soaker hose most of the water goes down instead of around the trunk.