Chief, I think we've talked about this before. The 18th Georgia was never referred to as the "3rd Texas". That error came about sometime in the 20th century by someone who misread or misinterpreted the chronicles of Hood's Brigade. The 3rd Arkansas was the regiment referred to by the Texas regiments as the "3rd Texas" as a sort of honor to their Trans-Mississippi comrades.

It had nothing to do with the numerical gap between the 1st and 4th Texas (why not the "2nd Texas"?). It was simply a function of tweaking the Arkansans' 3rd Regiment title by inserting "Texas" in lieu of "Arkansas".

There is nothing in the contemporary or postwar records of Hood's Brigade that links the "3rd Texas" moniker to the 18th Georgia. That is a 20th century invention (or error, actually).