Day and Herriott joined the Saints Senior Academy ahead of the 2011/12 campaign, while 17-year-old Stephenson has been training with the full-timers this summer following some run-outs in the Wanderers towards the end of last season.
They are part of a 26-man squad that will play the New South Wales Academy, Australia Schools 'A' and Australia Schools in the space of a week.
Fletcher says that all of the players who are making the trip will see some rugby on the tour.
"Everybody will get a start," he said. "Everybody will get the same opportunities because at this age that's what it's all about. We've got two tough games before the Test match against Australia. We're expecting three tough games - Australian youth rugby is strong, but our guys will be looking to do well.
"We're there for two weeks and will play three games. The first one is just five days after we arrive so it's important for the lads to acclimatise. We'll meet up at Twickenham and go straight to bed in the afternoon and wake up at midnight ready to train at 1am to prepare ourselves."
ENGLAND U18 SQUAD TO TOUR AUSTRALIA
Dominic Barrow (Prince Henry's Grammar & Leeds Carnegie)
Joe Buckle (Bryanston & Bath Rugby)
Jack Clifford (RGS Guildford & Harlequins)
Luke Cowan-Dickie (Truro College & Exeter Rugby)
Matt Hankin (Haileybury College & Saracens)
George Sandford (St Paul's Catholic College & London Irish)
Danny Herriott (Moulton College & Northampton Saints)
Tom Jubb (Oaklands College & Saracens)
Max Crumpton (Oaklands College & Saracens)
Tom Price (Wyggeston & Queen Elizabeth 1st & Leicester Tigers)
Kyle Sinckler (Epsom College & Harlequins)
David Sisi (St Paul's Catholic College & London Irish)
Tom Smallbone (Harrow & London Irish)
Scott Spurling (Harrow & Saracens)
Jack Arnott (Ivybridge College & Exeter Chiefs)
Alex Day (St Joseph's College & Northampton Saints)
Sam Hill (Ivybridge College & Exeter Chiefs)
Will Robinson (Twyford College & London Wasps)
Mark Jennings (Myerscough College & Sale Sharks)
Sam Jones (Ivybridge College & Exeter Chiefs)
James Lightfoot-Brown (St Paul's Catholic College & London Irish)
Jack Nowell (Truro College & Exeter Chiefs)
Henry Slade (Plymouth College & Exeter Chiefs)
Tom Stephenson (Moulton College & Northampton Saints)
Anthony Watson (St George's College & London Irish)
Marcus Webber (Manchester Grammar & Leeds Carnegie)