Well... if T represents a number and 3T represents 3 times T, then any number is the answer, as any number added with itself 3 times results in 3 times itself (obviously).
If T represents one positive digit (0-9) and 3T represents a 2-digit numer (if T=2 then 3T=32, for example) then this problem is impossible as its stated (no matter in which base it be, decimal, binary, hexadecimal, whatever).
So, i strongly suspect that its not a math problem, only seems like one.