bad idea. you already need to upgrade the size adding a third and hoping they’re gonna get along isn’t a good plan. what happens if the two gang up on the one the first day home? and even having two that aren’t bonded in a cage is really dangerous. all of this screams danger to me. even bonded birds can get into huge fights. that’s not even touching the fact that three is a bad number, two gang up on one, two are always closer then the third. and adding a third bird adds a lot of additional strains. if these birds really aren’t bonded the stress of a new bird in what they perceive to be their home could stress them and cause fighting, what are you gonna do about quarantining? it sounds like there wouldn’t be a quarantine plan which would also be pretty dangerous especially if this is a bird in need and not a bird from a breeders. if you don’t have space for another cage DO NOT GET IT.