Only two with BT but you can get three if you use AUX and wires. Buy two splitters and attach one splitter to the next. Now you have three female 3.5-inch connectors and you can connect wires to all three of the Revolves. Not perfect but works.
As I understand it, Bose had to go outside the BT 4.x standard to do Party mode since BT 4.x does not include functionality to connect to multiple simultaneous devices. I believe I read something about BT 5.x that it does provide support for that. So if that is true, three (or perhaps more) should be easier to implement in the firmware for such devices. However (and I do not work for Bose), I do not believe that Bose intends to upgrade the firmware of its current devices to BT 5.x and so we will likely see that functionality only with the next generation of Bose BT devices.