Stegmeier Flowmaster 3 Commercial FM3DCUP 3" Deck Drain Coupler Gray, PVC. Flowmaster 3 Commercial is a 3 removable top drain that features a double wall base and a snap in top (PVC). When installing this drain the tops are staggered past the joints making the joints very rigid. Because the Flowmaster 3 Commercial Deck Drain tops are removable this drain is easy to clean and the tops can be replaced if necessary. Flowmaster 3 Commercial Deck Drain is used on many commercial swimming pools, patios and other areas where large amounts of water need to be removed.