I'm looking for a gas tank for a 73 rt3 360. Are the tanks from other models the same or does this use a specific tank? I see tanks on ebay from 100's, 125's and 175's etc and they all look to be the same tank. 

Tanks for 1972 and 1973 DT2/3 and RT2/3 are the same dimensions and fitments. Others could be made to fit, but wouldn't be "right".

With my anorak on and my rivets being counted, the RTs came, I think, with a sticker just below the fuel cap, advising to engage the decompression valve before starting.

Did they all have the hinged flip cap or did some screw on?

My RT2 I bought, had a tank with a screw on cap. GR8uncleAl pointed out it was a MX tank. I now have a DT250a fuel tank on it. Looks close enuff, yet, for the ones with a trained eye will tell me it’s not correct. How much will I let it bother me?
Sneezles61

Just to be clear, 100, 125, or 175 tank will not fit a 250/360.

Same with mine, its an RT3 with a screw on cap, presumably from an MX. Looks identical except for the cap. I remember back then guys used to try to make an MX out of the enduros. I plan to paint it all back  to original Baja Brown. Tank looks close, might just keep it if I can't find a reasonably priced original. . 

Good to know as they all look the same on Ebay. So does a 250 tank fit a 360 then?

The years from 68 through 1971 are often called "badge bikes" due to the plastic badges screwed to the sides of the tank.  In 68 there was only a 250 and the tank mounting was unique...  but starting in about 69 through 71 the 250/360 tanks were interchangeable except for paint color.

Then in 72-73 the tanks lost the badges and got the "flip top" gas cap.  Tanks are interchangeable from 72-73 between the 250 and 360 except paint colors and decals changed depending on year and model. 

In general, the 250 and 360 "frame" parts like gas tanks are interchangeable within the same "series"...  In your case a 1972 or 1973 250 (DT2, DT3) tank or a 1972-73 360 (RT2, RT3) tank will fit your 1973 RT3 exactly.

There is some forward and backward compatibility sometimes too...  and as other have said there may be a "MX" tank that fits.  To stay "original" you'll need a 72-73 RT or DT tank though.