The third harmonic is given as 3fo, where fo is the fundamental frequency.
The fundamental frequency is the lowest frequency that can occur in a pipe. In an open pipe, both even and odd harmonics occur which are multiples of the fundamental frequency fo. Hence the harmonics in an open pipe are; 2fo, 3fo,4fo..... etc.
For the third harmonic; 3fo = 3 (440 Hz) = 1320 Hz
