> Server B is a SQL 2005 warehouse on prem, and server c is another SQL 2005 server on prem.
SQL server 2005 is out of supported for a long time. Suggest you upgrading your SQL server to a newer version. Don't let your infrastructure and applications go unprotected.
> Is there any way to enforce encryption between AWS and B but allow C to talk to B?
No. If you want to connect to SQL server instance(enable encrypted connections)on server B from server C, you need to copy either the original certificate or the exported certificate file from server B to server C.
Refer to MS document Enable encrypted connections to the Database Engine to get more detail information.
If the response is helpful, please click "Accept Answer" and upvote it, thank you.