500 gal is plenty to have more than one pair of clowns (of the more docile variety like Pink Skunk, Ocellaris or Percula) in more than one natural hosts. I have several pairs in my 420 in the past and in my 320.
The main thing is separate the anemones to three separate areas then just add the clown fishes. The dominant pair will get their pick of anemone and the other two pairs will get what is available.
Often there can be some chasing but they will eventually separate out.
It is best to not have three females right off the bat. This way, at the beginning, you have a dominant pair. The other two pairs likely will develop and mature. However, if the anemones are too close together, the dominant female may keep the others juvenile clowns from mature (even different species).
I have keep breeding pairs Ocellaris and Pink Skunk in my 420ish tank (96X36X30) about 18 years ago. Also have multiple pairs of Ocellaris, Percula and Hybrid of the two in my current tank that just got zapped by Harvey