NIBCO 4 in. x 3 in. PVC DWV Hub x Hub Reducing Coupling C4801HD43 - The Home Depot
Amazon.com: FangKingNo10pcs 3/4" White PVC Pipe Coupling Adapter Pipe Fittings (Slip to Slip) Schedule 40 Easy to Install,and High Tensile for Home or Industrial Use (1 Pack of 10pcs) (0.75 Inch) :
3 Inch PVC Coupling Slip x Slip [429-030] - $6.68 : The Spud Gun Technology Center, Spudtech
6203005 PVC & Accessories 3" SCH40 Long Line PVC Coupling