Of the three, IAD is the least prone to bad weather that leads to a disaster.

The next consideration would be seat arrangements. The 787 is 2-3-2 in PE while the 767 is 2-2-2. Not sure the size of your family, but one configuration may be better than the other.

Finally, the noon-ish flights from IAH-EWR/IAD are widebodies. They are less prone to delays/cancellations and a more comfortable ride (though they don’t sell PE seats officially on those legs).

As a curve ball, United will sell you IAH-Frankfurt/Munich-CDG. I prefer the longer flights over the water so that I can rest and be ready on arrival. Those international flights are also very unlikely to be cancelled, even in bad weather.