How does UNAS Pro handle adding disks?

Pulled the trigger on a UNAS Pro and ordered three WD Pro drives to start me off but they all arrived at different times. I’ve been away visiting family lately so only one drive and the UNAS had arrived by the time I left. I put that in to get through set up but the rest now have been delivered.

What will happen when I put the other two in? Will it expand the volume (taking into account the usual RAID capacity stuff) or will it reformat the original first disk first? I haven’t used it yet so do t mind either way but I was just wondering what happens?