If you can't find a 3x3x2 you may have to cut the 2" line back toward the gray tank, install the new 3x3x1-1/2 as shown, with new blade valve, then connect up the old 2" to the new 1-1/2" with a reducing coupling. It may slightly reduce the flow rate on your gray drain, but there's nothing in the gray that should clog on the reducer.
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element