Sn3 Couplers
This page covers couplers that are in use today by Sn3 modelers, and hopefully provides you with some guidance as to which one or ones you would like to use in your own models.
In the past, most Sn3 modelers used the Kadee #4 coupler. This was an HO-scale coupler that is now discontinued.
A Kadee #5 (similar sizes in other brands) is also a close match for a Sharon coupler (whose coupler head measures 13-1/2" tall in the real world).
If you are an old-time modeler (as in modeling era, not your biological age!), Wiseman Model Services produces link-and-pin couplers that fit in the Kadee #4 draft box.
Gauges
The late Tom Troughton created a web page about how he created his own Sn3 coupler height gauge.
There is also a coupler height gauge available on the CESCO on the 3DP Train web site.