Harrison, 19, has been at Franklin's Gardens as a triallist since December. The former 1st XV captain of Rotorua Boys School - whose fellow alumni include internationals Dylan Hartley, Liam Messam and Craig Newby - made his Saints first team debut in the Aviva Premiership Rugby win over London Wasps.
He backed that up in the successful trip to Bath a week later and has been a regular in the Wanderers team which has reached the Aviva 'A' League semi-finals.
Seventeen-year-old Stephenson is one of the highest rated young players in the country. The Oxford-born centre is a student at Saints' partner college, Moulton College, and has come through the club's Junior Academy system.
Already a regular at Wanderers level in the Aviva 'A' League, Stephenson recently showed his leadership credentials by skippering England Under-18 to a win in Scotland, and scored a try in the FIRA/AER tournament win over Georgia this Easter.
Allcock, who has just turned 18, may not be as familiar a name to Saints supporters, but as a soon-to-be graduate of Sedbergh School he will be following the likes of Phil Dowson and Tom Hicks in pursuing a career at Franklin's Gardens. The flanker has already experienced Wanderers rugby this season, including in the recent win over VVA-Podmoskovye.
Director of rugby Jim Mallinder said that all three players demonstrated the talent which is coming through the Saints Academy system.
"All three players are young, but all of them have shown in training and in matches that they have the ability to fit in with the full-time squad at the club," he said. "Teimana has thoroughly deserved his opportunities at first team level and has shown that he can play at that level.
"Tom has been a standout performer for the Wanderers, has done well with England Under-18 and is a player with a lot of potential. And Jack has impressed us when he has been with the club, too.
"All three - as with all the other players in the Senior Academy - have shown they have got what it takes to earn a full-time contract. Now it is up to them to demonstrate they can take the next step and establish a long and prosperous career in professional rugby."
Harrison is already training full-time at Franklin's Gardens, while Stephenson and Allcock will join the squad upon the completion of their academic commitments.