(b)"average adjusted total income" means,—(i)if the assessee is assessable for each of the three tax years immediately preceding the relevant tax year, the arithmetic mean of his adjusted total income over those three tax years;(ii)if the assessee is assessable only for two of the said three tax years, the arithmetic mean of his adjusted total income over those two tax years;(iii)if the assessee is assessable only for one of the said three tax years, his adjusted total income for that tax year;