Most scripts, dictionaries and decisions are made based on usage. The use of D.R. has been universally accepted. For example, send a letter to a city like Santo Domingo, DR, where will it go??? Dom Rep. Send it to Santiago, DR, where will it go? Dom Rep. Send it to ST james, DO, guess where it will go? DOMINICA. Look at letters, stamps, chips, you will not see DO for Dom Rep, but you will see it for Dominica. You add a 3rd letter, DOM and 100% of the time it will go to Dominica.

In Spanish is always the same RD (Rep Dom) and in Spanish DO or DOM will always be Dominica.

I do not believe anything but DR will work, it has been used for over 100 years. And nobody should confuse it with DO, Dominica. If you need 3 letters you can allow next to it an x or a 0, but that may just cause confusion!!.

How about a simple idea, those who want to use a 3 letter code allow for certain countries or territories the use of only two letters when history and use have made the 2 letter code causes immediate recognition,hwile the addition of a 3rd letter causes confusion: DR, PR, UK GB.......... very well known, others less known, adding a 3rd will only confuse, exp- C.R., Costa Rica, not that well recognized, but add anything to it and you will really confuse the reader.