PTFE Solenoid Flange Gasket
The 3-way valve body has o-rings that seal it to the group head, so this bit isn't strictly necessary. Eventually, though, those o-rings will fail, because failure is one of the major categories of o-ring operation. When that happens, rather than drop $30-80 on a new valve body assembly, shim one of these gaskets in and call it a day. You might also need new bolts if the existing ones aren't long enough.