For a 3 inch ABS pipe outlet on your RV black water tank, the 3" Flex Black Rubber Tank Fitting (Part #93 500125) is an ideal grommet. It fits a tank hole size of approximately 3 7/8 inches and provides a flexible, leak-resistant seal when inserting a 3 inch ABS pipe. If the ABS pipe is stuck and cannot be removed, you can cut it flush with the tank and use a rubber coupling or a Fernco-style flexible connector to insert a new pipe inside the old one. Alternatively, if the tank hole is damaged or oversized, a bulkhead fitting or spin-weld fitting might be necessary, although spin-weld fittings typically require specialized equipment and professional installation. Be sure to measure your tank hole diameter accurately to ensure compatibility with the grommet or fittings you purchase.