You say you have 3 phones.
Is that 3 phones plugged into 3 different sockets? Or 1 cordless phone with base unit and 2 additional handsets?
If its 3 sockets are they individual sockets or are 2 of them wired to 1 socket?
What you need to do is test everything, piece by piece.
Unplug everything everywhere.
Plug a filter into the main socket. Plug a phone into the filter. Try the phone.
Try the other 2 phones in turn (assuming they are not cordless phones)
If that doesn't work, repeat but using a different filter.
If that works, take note of which bits work, then unplug everything again and repeat with the next socket, then again with the third socket.
Then reconnect socket 1 and then add in socket 2, then socket 3.