It doesn't say who's 3rd choice, but considering Wildsmith has only played once for England U20s, with Walton having played 4 times for the U19s and 8 for the U20s, it's pretty clear who's further up in the pecking order.
This is made even more impressive with the fact that one of the 6 championship players listed is the 3rd choice goalkeeper, which makes him pretty much irrelevant to the squad as he's very unlikely to be used. We basically have 3/5.