The full sized 8 head traffic advisors with the same housing are easier to find, you can cut them down and make a pair of three head units and one single head unit. You can't quite get a pair of 3's and a 2 head because you lose material cutting. I use a band saw but if you are careful a hacksaw works. The guts are easily pulled from a 2 head strobe model, then you add a halogen head in the center from the traffic advisor. You can keep the end caps from the 2 head version and from the traffic advisor. That's how I plan to get mine (I made a few previously).
I will build a second one when I build mine if you want (no projected project date like most of my ongoing builds).