Well, now I've made a triple cockring, and as usual, I'll give you a guide on how to do it...*S*
Which sizes YOU are going to need, for your creation, is of course depending
on how you are build, but I'll give you the measurements for the rings I have
used, as a guideline.
Remember that the rings must be 'rather tight', although not TOO TIGHT !
Hére's the sizes I've used:
Ring 1; around 'the whole thing': 45mm
Ring 2; around the ballsack: 39mm
Ring 3; around the penis: 30mm
The sizes are internal diameters (metric).

The rings are purchased in my local 'sailor shop' who are usually quite well
equipped in stainless steel rings, in all sizes and shapes.
The 'connections' are made from an 'edging band', I purchased at 'Stof
2000'. I've used a red sewing line, in order to make the sowings visible in
the pictures.
By the way, there's no one stopping you from using i.e. a piece of black leather,
to make the connections, or which ever material you fancy.
Triple cockring, as accessory to my black stretch jeans: