So companyA.com will no longer exist but how are you going to differentiate companyb.com's users and companyC.com users?
companyA.com in the interim can forward to companyB and CompanyC all email or create a rule that returns all messages with a message to use another email address
So both new companies will have to set up their own DNS and email solution
a mx record can only point to one address
you can have multiple mx records if you have redundant smarthosts
if you have
mx companya.com companyb.com and
your results will be unpredictable it might round robin 1/2 going to each mx record.
companyb.com accepted domains companya.com companyb.com
companyc.com accepted domains companya.com companyc.com
this will cause a lot of user doesn't exist ndr's when someone is at companyc.com has mail delivered to companyb.com
shadow_transport = local_copy_incoming
shadow_condition = ${if eq {$domain}{domain.com}{yes}{no}}
(You will need some active server for this).