yeah, I think thats the best idea. I just measured it...just about 3.5 OD.
what confused me is that the bayonet fitting says right on the label: 3" O.D.
methinks they're mistaken. (or are they?)
standard "3"" pipe is 3" on the INside; and as you said, 3.5 OUTside.
Instead of a regular "sanitary T", which most often have all "hub" (female) connections, you might look for a "sanitary street tee"...which has hub on one end, and "street" or slip on the other. (like a straight piece of pipe) then you could just cut it anywhere you need, and glue-on the bayonet ring.
might also consider putting on a valterra type, instead. the Thetfords are more rugged...but the valterra slinky fittings are more readily available...on the shelf at any Walmart. (for emergencies).
can't find abs pipe in hardware stores around here, as its not "code" for houses. everything is pvc. but the sizes are the same.