Upgrade from SEPM 12.1.2 to SEPM 12.1.4

Hello,

We have 3 SEPM servers that are in one site connected to one database (SQL 2008) .  We want to upgrade the SEPM servers to the latest release.  Because we have so many clients that are depending on the SEPM servers, we want to be cautious, and only upgrade two SEPM servers in case anythng goes wrong, we will have 1 SEPM server still on the old version.

Does the logic above make sense and will it work?  Can we have a mix version environment of SEPM servers in the same site connecting to one database? Or will the 3rd server not work?

Thanks,