The air column of a flute acts as a pipe 0.68 m long, open at both ends…

$f_1 = \dfrac{v}{2L} = \dfrac{340\ \text{m/s}}{2 \times 0.68\ \text{m}} = 250\ \text{Hz}$. An open pipe has every harmonic, so the next two are $2 \times 250\ \text{Hz} = 500\ \text{Hz}$ and $3 \times 250\ \text{Hz} = 750\ \text{Hz}$.