The correct symbol for an atom of tritium?

3H is probably the most common. Technically 31H (the 3 and the 1 should actually be "stacked", but HTML doesn't really allow that) is correct as well, though the 1 is redundant (the H already tells you it has 1 proton).

Since T is not used as a symbol for any other element, you'll occasionally see that as well. However, it's not "official".