How common/rare is it for a doe to have triplets?

Depends on the ranch location in the state and rainfall. Many more are born than actually seen due to predators and lack of cover/forage for the doe to support them. Seen more in South Texas than anywhere though. Huge swings in nutrition in rain lead to more of them making it to age they can be seen. IIRC Dr. Beasom from CKWRI stated one time about a study done in the 80's in La Salle County that they did some habitat work, food plots and protein supplement on a very large ranch by pasture to do research. One pasture they drug a heavy piece of railroad rail behind a dozer to knockdown 20% of it in one strip. They had low deer density but still shot 5 does in that pasture that spring. 3 of the 5 does were carrying triplets and the other 2 had twins in them. They did 20% in a strip next to the first strip late that spring. Deer pop was moving up noticeably. Did the next 20% the following year and then unexpectedly the deer population started to drop off. 3 yrs of laying over 60% of the brush/cover and lack of any normal rainfall led to less cover(fawns and adults) and regrowth is what Dr. Beasom thought was happening. They had planned to do 20% per year for 5 yrs then start back over on the first years 20% area in year 6 to continue the research but the lack of rain and regrowth stopped the research.

^^^What the MAN said^^^ Had a doe that always had trips for 3 years. She is now gone, have not seen her in a year, but she had trips every year for those three years that we had her on cameras.

Twins are the norm but with good nutrition triplets often occur.