Steamdeck 3 being statistically impossible is a joke about Valve having an aversion to releasing a #3 of anything.

Half life 1, 2… hm.

Ok we’ll make three HL2 episodes to follow that up.

Ep I. Ep II. Uhhhh… and let’s stop there, just like forever I guess.

Portal 1. Portal 2.

Left4dead. L4D2.

Team Fortress. TF2.

Counterstrike. CS2. Oh shit we’re releasing a third one, we might have to use the number three finally, oh no… I’ve got it: CS:Go!

It is a joke about Valve only rarely making the third installment in any of their series (e.g. Half Life 3 not existing) Edit: specifically including "3" in the title - the actual number of games tends to be higher but with different names.