The farthest I drove this summer was a bit over 700 miles to Colorado Springs. We left about 5:30 Am and got in about 8ish. Made for a long day, but with the DVD player and the I-Pads, the kids (3 boys) didn't gripe at all.
Our youngest son (4 1/2) is a road warrior, and has always been able to handle 10+ hour drives. The only time he got cranky was when we were coming back from Charleston and we were on the 2nd day of the drive. The last 100 miles, he was a little testy.
I might suggest altering your day 3 schedule to stop at Balmorhea State Park in Texas. They have a 2 acre spring fed swimming pool.
Courage is the feeling you have right before you fully understand the situation