score:4
This phrase here means everywhere.
Specifically, probably the Trailokya, the realm of desire, form and formlessness. The realm of desire is hell and earth. The realm of form and formlessness are different types of heavens.
ref: https://en.wikipedia.org/wiki/Trailokya
The 10 realms, 10 directions, a three times are all common stock literary phrases that meant everywhere and everything and all times.
And I've lost track of how many ways I've seen the realms sliced up, into the 4, 6, 10 realms, and now with this question, 3!
Upvote:3
http://www.netplaces.com/buddhism/karma-and-worldview/the-three-realms.htm
Also called
http://www.accesstoinsight.org/ptf/dhamma/sagga/loka.html
Since that encompasses the whole totality of the cosmos, the buddhas in question are all the supremely enlightened ones -- clear as day to a purified mindstream.