What happens if someone named in a trust refuses to participate?

Slightly confusing, I know. Sorry.
For example, a parent dies, leaves monies in a trust to be divided equally among the three children. Much paperwork to be done. One child decides to simply not do the paperwork or deal with it at all. What happens to that third? Is there a statute of limitations before it moves on? Does it go to the next of kin of the beneficiary who didn’t acknowledge it? Does it revert to the two who did?

No other instructions were given.

This isn’t a thought exercise, I want to know if anyone actually knows.

General Q.