diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 111261832..c47346a4b 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 756c2dbde..3dad4e954 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": {