diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 111261832900d107d33f68fcc3c9841548328842..c47346a4bf21a3c11bfb56b3d0aae36c622b2026 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -119,6 +119,7 @@ KNOWN_ROOM_VERSIONS = {
     RoomVersions.V2,
     RoomVersions.VDH_TEST,
     RoomVersions.STATE_V2_TEST,
+    RoomVersions.V3,
 }
 
 ServerNoticeMsgType = "m.server_notice"
diff --git a/synapse/rest/client/v2_alpha/capabilities.py b/synapse/rest/client/v2_alpha/capabilities.py
index 756c2dbdece80238e9fec2b50e58d4049993eede..3dad4e954279f514372e5cf124cdcfb9526b0290 100644
--- a/synapse/rest/client/v2_alpha/capabilities.py
+++ b/synapse/rest/client/v2_alpha/capabilities.py
@@ -53,6 +53,7 @@ class CapabilitiesRestServlet(RestServlet):
                             "1": "stable",
                             "2": "stable",
                             "state-v2-test": "unstable",
+                            "3": "stable",
                         }
                     },
                     "m.change_password": {