Until their first summer is over, soak them deeply every few days. You need to get the soil wet way down deep so the roots will go deep and be safe from drying out in the occasional hot spell like the one that is coming.

Apply 10 gallons in one day, but slowly so it has a chance to soak in deeply. Do that twice a week until fall, then taper off. Next summer, a deep soaking every 2 weeks unless it's over 105 should be enough.

I suspect that the third plant just needs more water than it is getting.