Program his extn to ring with DEE turned on to twin to his mobile, this sorts when he is out and about.
Set up a Hunt Group with the three extns in as a list, forward all calls from the phone that rings to the Hunt Group.
All three phones will ring as a hunt group call will not follow an individuals call forwarding.
So it seems a bit backward but when he is in set the call forward, all phones ring.
When he is out turn the call forward off, his primary handset and mobile will ring.