Active-Standby configuration - more than 2 devices?

I'm with Stephan here. The third device addes unnecessary complexity. I've done similar migrations myself using more or less the same strategy as Stephan and they've worked without a hitch. 

Just make sure to have some kind of verification script before removing the passive node from forced offline to make sure that the connectivity in terms of cables etc works fine. Usually this means to ping the server side nodes, the default gateway and the peer device self ips.

Good luck!