Forward incoming calls to primary line

I’m trying to forward incoming calls like so:

111-222-3333 - Main Number
111-222-4444 - Secondary Number
111-222-5555 - Tertiary Number

Incoming calls to 111-222-3333 go straight to the ring group for all phones
Incoming calls to 111-222-4444 or 111-222-5555 attempt to ring back to 111-222-3333, and if that line is busy for longer than X seconds, it rings to the ring group for all phones

I want to set this up so that when call forwarding is enabled VIA the phone provider (on their end), incoming calls are forwarded back to the main line which forwards to whatever destination is necessary.

Hopefully someone has some way to resolve this.