Azar went on a three-day tour of London

In this case three-day is used as an adjective to describe the tour. Alterntively you could say, "Azar went on a tour of London for three days."

I have changed 'to' to 'of' because I assume you mean that Azar went to London, then took a tour lasting three days. A three-day tour to London means that the entire journey, getting to London and coming back, took three days. 

I hope that helps.