Thetford Curved End for 3" Hose
This Theford pronged hose end is designed to fit only around Thetford bayonets (VTS-440) on your trailer. The slip part fits inside a standard 3" flexible RV sewer hose and is secured with a hose clamp.
You might also consider VTS-441, which is a straight version of this adapter.
Thetford PN 01665.