I don't hink 'in company' is in the book. But a little common sense on the radio usually works. And most ATC seem to prefer talking to a 'combine' - to use the RAF word - which we never did, than in talking to 3 aircraft any number of which might squawk.
Then use your normal calls simply adding in that you are splitting apart. Or that one of you has gone AWOL and is no longer party of the formation. As the radio man you simply have to use the normal calls with the additional information that either ATC or the otehr aircraft in the formation need.
"G-ZZZZ approaching blah for joining information. the other aircraft will leave formation and contact you direct for landing clearance" as an example when you are splitting up on arrival.