Those three players have been the best three players this year and it's not even close. Most of the big finals we had so far this year featured at least one of the three and quite often two. The Madrid one featured Sabalenka who defeated Swiatek for the trophy and after the match, Mouratoglou took to Twitter to anoint them the big three:
"WTA has its big-3! Aryna Sabalenka, Iga Swiatek, Elena Rybakina. The third one doesn’t have the ranking just because her points from winning Wimbledon haven’t been displayed."