You know whose turn it is to dive next, and who is safety for whom.
You are safety for the person after you
Suppose divers A, B and C are on a buoy.
A dives first, then B, then C (alphabetical order).
A is safety for B, B is safety for C, and C is safety for A.
Do your safety from the surface
From the surface, if you notice a problem, you can alert the divers on other buoys, call for help, and pull up the line.
If there are three of you on the buoy, the third person can provide additional safety at depth if the diver requests it.
This third person is welcome to refuse to be safety at depth if they don't feel comfortable doing so, but if so, they should try to find someone else to replace them.