I'd say any 3 wins within a 1 year timeframe should count.
For example, with the current schedule, Vuelta 2010/Giro 2011/Tour 2011 would count... the end of the 2011 tour was less then a year from the beginning of the 2010 Vuelta.
But Giro 2010/Tour 2010/Vuelta 2011 would not count. The Vuelta was more then a year after the Giro.
For example, with the current schedule, Vuelta 2010/Giro 2011/Tour 2011 would count... the end of the 2011 tour was less then a year from the beginning of the 2010 Vuelta.
But Giro 2010/Tour 2010/Vuelta 2011 would not count. The Vuelta was more then a year after the Giro.