The numeral 3 is usually omitted from figures since it is so common. So although it exists in the chords 5/3; 6/3; 7/5/3; 6/5/3 and you'll usually only see it printed as the tail end of a 4-3 suspension (see suspension chapter).
The numeral 3 cannot be abbreviated from a 6/4/3 chord (otherwise it would be indistinguishable from a 6/4 chord). This is a rare chord in 17th- and early 18th-century harmony, but became progressively more common in the 18th-century.