diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index c47346a4b..111261832 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -119,7 +119,6 @@ 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 d73153ccc..61319a7a2 100644
--- a/synapse/rest/client/v2_alpha/capabilities.py
+++ b/synapse/rest/client/v2_alpha/capabilities.py
@@ -38,7 +38,6 @@ class CapabilitiesRestServlet(RestServlet):
                         "1": "stable",
                         "2": "stable",
                         "state-v2-test": "unstable",
-                        "3": "unstable"
                     }
                 }
             }