The function of tRNA or transfer RNA is to bring into position at the ribosome the appropriate amino acid, as per the messenger RNA transcript. If the messenger RNA has the triplet AUG, then, the transfer RNA would carry the amino acid methionine into position, signalling the beginning of protein synthesis. Transfer RNA has the opposite of the codon on the messenger RNA which is called an anticodon. Base pairing rules are the same as in DNA except there is no thymine, so you must subsitute uracil. Each triplet would be "read" and the appropriate amino acid would be carried into position and linked to the previous one with peptide bonds. A polypeptide is the result which will detach when a stop codon is "read", fold and become a functional protein. The answer to the original question is that there are four possible bases in RNA, however, every tRNA has three bases corresponding to the codons on the messenger RNA.