I do a combination of
@Beanwood's and
@Peartree's. I only ever call them separately when they're in "work mode". I don't think it's unreasonable that they all come when you call just one during "downtime", as they aren't under orders at that point, so can do as they please (within the normal construct).
So on their beds, lots of reinforcement, and then call one off. If there is one more likely to break, I will call that one more often, maybe even after every other dog. If they break, when not called, I just put them back on the bed and make it easier. I've worked up to having this outside, being able to call a dog, throw a ball for them, dance around etc, have them walk away with me, do a few exercises etc, while the others stay settled.
I also sometimes do it when leaving the garden to go for a walk, although this isn't as strong. I sit them up one side of the gate, open it and walk through, then call them one at a time. To be honest, I haven't done this ins a while so it might not work anymore! My normal gate routine now is that they all stay my side (standing is fine) while I open the gate, then either I go through and then release them, or else I send them through ahead of me. in either case, they wait for me on the other side before we move off.