There was an article in the Daily Mail on Friday last week. Researchers from Stirling university took two groups, the first did 5x4mins hard 2mins easy, 3 times weekly on a stationary cycle. The second group did 60 mins steady 3 times weekly. After a month they swapped over. The researchers claimed the interval style training gave twice the improvement in power and performance. Researcher Stuart Galloway was quoted 'it is a case of training smarter. Amateur athletes tend to spend a lot of their training in the moderate intensity bracket which showed smaller improvements.'
This shows what is possible on a thrice weekly interval program for a month but it would be a mistake for an athlete training 5-7 days a week to do three interval sessions. They might get away with it for a few weeks but then they'd burn out.