I own three kitchens. Can one login open all of them without three passwords?

Yes, through the hub. Ask us to group your branches and name one owner login as the group's hub. On the branches screen the hub sees Open as owner on every kitchen in the group: it opens that kitchen's admin panel as its owner for fifteen minutes, with a banner counting down and Back to branches to leave early, and can do everything the kitchen's owner can except change that kitchen's own login email and password. A kitchen whose trial ended, whose payment failed or that is suspended does not open this way. Every opening is recorded under the hub's name. The Account, trial & billing tab in the docs covers the hub.