I would recommend getting in touch with your account team for this question. If you already have the hardware in place, you should be able to purchase capacity licenses regardless of hardware generation but Gen3 expansion nodes are in short supply these days.
thank you for a very swift reply to my query.
The end user has exist physical capacity in their Avamar installation.
Looking through channel express there appears to be no way to order capacity licenses for a Gen 3 installation since the move to Gen 4 and Gen 4S.