Reptiles, birds and mammals have something in common. What is it?

A  They are the first taxa to not lay eggs. Amphibians were the last taxon to still lay eggs among tetrapods, and today only birds still have that ability.

B  They are all Amniotes which means that, unlike amphibians, they produce a terrestrially adapted egg, which means that the embryos of amniotes were provided with their own aquatic environment. The embryo in amniotes is is protected by amniotic membranes.

C  They are all poikilotherms, able to control their body temperature. That is how they survived through periods of changing climate.