How good must those other two fullbacks be, if Reguilon is 3rd choice
Well Marcelo is one of the best full backs of his generation and Ferland Mendy is better than Reguilon - so they're pretty good.
Whilst I think Marcelo is no where near his best and I'd probably have Reguilon over him, Zidane strikes me as someone who's pretty loyal to the guys who have served him well - which is why Kroos, Modric and Casemiro still dominate his midfield, Benzema is still up top, Carvajal is their right back and Varane and Ramos are still their centre back pairing.
7 of those started in Zidanes first CL win 6 seasons ago.