I'll eventually get all three, but as to the order I'll buy them in...
Unless MS makes another huge screw up or the PS5 completely outclasses it, then I will start off with the Xbox 4. Everybody I play online with is on Xbox Live, and I prefer Xbox controllers.
I'll get a PS5 once it gets a big price cut. I like Sony's exclusives, even if I don't particularly care for their hardware writ large.
By necessity, I'll end up getting the Switch's successor last. It probably won't be out until two or three years after the PS5 & Xbox 4 are released, and I'll probably have both of those by that point.