A SQL Server instance can host multiple mirroring partnerships. Mirroring is between databases and databases are hosted on SQL Server instances, so choosing a name like 'DBx' for servers is a rather poor choice. Lets call the instances SRV1, SRV2 and SRV3 and the databases DB1 and DB2. What is possible is:
- to have the database DB1 mirrored by SRV1 (primary) and SRV2 (secondary)
- to have the database DB2 mirrored by SRV3 (primary) and SRV2 (secondary)
But the question is Why do you use mirroring for?. Primarily is a High Availability feature. The question is: will the SRV2 instance be able to take over the primary role for both DB1 and DB2 in case of failover? Nothing prevents both SRV1 and SRV3 from suffering a problem at the same time. Also, if SRV2 suffers a disaster, you will have to come up with a backup plan to restore the HA for both DB1 and DB2. Are you prepared to handle all these cases?
Would this configuration be possible using separate instances on DB2?
Yes, but is not necessary.