I've been trying to figure out which unit this bayonet's 3D.RGA markings refer to but, the more research I do, the more confused I become. Can anyone help?

Unit marking on Pattern 1888 bayonet

Some of the possibilities that I've come up with are No.3 Depot (Heavy and Siege) Plymouth Royal Garrison Artillery, No.3 Depot (Coastal) Plymouth RGA, 3rd Mountain Artillery RGA and 3rd Mixed Brigade RGA. Would any of these be correct?

Thanks!