Rule of thumb to compute global rank for a cluster of domains

Let say that domain A has Alexa rank a, domain B has Alexa rank b, domain C has Alexa rank c.

Is it correct to say that the global rank if you combine the 3 domains together, would be approximately 1/(1/a + 1/b + 1/c)?

This question is interesting to advertisers trying to achieve some reach numbers by advertising on niche websites, for which no traffic data is available except an Alexa or a Quantcast rank.