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 and Diane (RIP Lincoln, 21 FEB 22) RVM103 NHSO
Fulltimers since June, 2012
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element