Active-Standby configuration - more than 2 devices?

Hello Asura2006, BIG-IP sync/failover device groups support up to 8 devices so this should be possible on paper.

I'm going to add a little consideration here, while it's true that this cuts out the risk of primary unit malfunctioning, it's also true that it requires additional steps to re-configure HA  which can possibly result in being more of an issue (and possibly very time consuming if issues manifest) than just powering unit off. 

The "scary step" will be adding the new device to trust domain, I've performed this multiple times back in v12 for hardware refresh activities and I recall that this step was often not so smooth, I always had to delete and re-build trust from scratch with impact on HA (to prevent split brain when doing this, I forced offline both the standby and the "new" units so that only Active would see traffic, and I only plugged data interfaces back after I re-configured HA trust). 

Once appliances trust each other, just add the new unit to existing device group. 

You can control the way that the BIG-IP chooses a target failover device. This control is especially useful if you're using heterogeneous hardware platforms that differ in load capacity, because you can ensure that when failover occurs, the system will choose the device with the most available resource to process the application traffic.