text reads

Action: Choose up to 3 Spell or Artifact cards in
your discard pile. Until the end of the phase, you
may play those cards as though you were playing
them from your hand. If any of the chosen cards are
event cards, put them on the bottom of your deck
instead of discard pile after playing them.

now lets say i pay for chant of thoth with domain, so it is drained. asuming i have only 3, does this mean i have 2 others to play the spells from my discard? I pay for these with my other 2 domains right? i asume im reading it right but i find it weird that u are alloud to choose 3 spells or artifacts and not 2 because u wouldnt beable to play them with the remaining domains.

let me know how u play this card

Yes, you understand it correctly.

Remember that some cards have a cost 0, so you don't need a domain to drain. Also there are mechanisms (such as Eldritch Nexus) that allow you to create a fourth (or more) domain.