I thought it was an average of the three, but both my DS and DD's VQN score equaled their Quant scores and different from their nonverbal/verbal scores.
For example, my DD's {V,Q,N} scores are something like {133, 141, 128}, but the VQN is 141.
Both kids' Nonverbal and verbal scores were 10+ pts below their Quant scores, so it's bewildering to me how a VQN is calculated.