Automatic failover will happen. After this, application will be directed to new principal server(earlier it was mirrored) and will find only one database (in your case). Other two databases will remain in old primary server which will be unreachable to your application.To solve this, you have to setup a alert for failover event, and force your other databases too to failover.
Failover will result in running "recovery" in mirrored database. Downtime will depend on how much transactions to be recovered.