The given sequence is a combination of two series:
i.)Y,T, O and ii.)B, G, ?
i. consists of 2nd, 7th, 12th letters from end of English alphabet,while
ii. consists of 2nd, 7th, 12th letters from beginning of English alphabet
So, the missing letter in ii.) is the 12th letter from the beginning of English alphabet,which is L.
