This could be an intentional design feature regarding power loss to critical reactor control systems.

Rig N+2 control rods with an actively powered electromechanical retaining mechanism. In any kind of serious electrical issue occurs on the protected circuit, all 3 drop automatically.

Yeah, on one of the reactor designs I was familiar with, control rods were bundled into groups which were managed as a set and were intended to be operated in different sequences over core life.

So when I read "3 control rods hit the bottom" what I hear is "there was an unexpected group scram", but I don't know enough about this reactor design to know if that's actually what happened, or whether it was 3 unrelated control rods instead.