Found out my long time crush is dating...

Obviously because he wouldn't be looking to break them up, so they have no reason to object! For him? "If you can't beat 'em, join 'em," so our fathers have said.

I'm not certain how true this is, but I can't imagine myself seriously considering a girl as a love rival.

I'm envious because I've been trying to get over this girl for the past 3 years without success. Reading about your situation creates this scenario in my mind where she falls in love with another girl and I find that scenario completely hilarious and acceptable. The thought of her sleeping with another girl is so much more tolerable than the alternative.

So her dating this girl isn't a real relationship?

No sorry, that's ridiculous.
It's not a matter of if she is a "worthy love rival" or not, she is dating someone else, full stop, end of discussion.

You display some worrying double standards here if you think that 2 girls can't have a real relationship. And the whole "oh 2 girls, isn't that exciting" mentality speaks more of perverted fantasy than anything else.