This question is basically, "how much will each company have ready at launch?" Consoles always sell almost all of their supply at launch. Even the Wii U was in short supply right after its launch. So, I think PS5 will have the most ready of the three choices.
A more interesting question would be which system will sell more at the 12 month mark. I think Switch will have sold the most in its first 12 months.
curl-6 bet me that PS5 + X|S sales would reach 56m before year end 2023 and he was right.
My Bet With curl-6
My Threads:
Master Thread, Game of the Year/Decade
Switch Will Be #1 All Time
Zelda Will Outsell Mario (Achieved)
How Much Will MH Rise sell?
My Bet With Metallox