Who are the Buddhas of the three worlds in Ryōkan's poems? In some of Ryōkan's poems such as this one he mentions the Buddhas of the three worlds


  While I gather firewood and wild grasses on this hill,
  
  the Buddhas of the three worlds
  
  Are also celebrating


Who are the Buddhas and what are the three worlds?
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!