For an open organ pipe, the n-th harmonic frequency is fn=n⋅2Lv.
The 3rd harmonic is v3=3⋅2L330 and the 6th harmonic is v6=6⋅2L330.
Taking the difference: v6−v3=(6−3)⋅2L330=3⋅2L330=2200.
Solving for L: L=2×22003×330=4400990=0.225 m = 225 mm.