Lewis Wing and Hayden Coulson are the latest players to report back to Rockliffe Park ahead of pre-season, along with Marvin Johnson.
Johnson committed his future to Boro with a new contract today, and soon after was joined on the training pitches by teammates Wing and Coulson.
That brings the Boro party up to six, with new signing Grant Hall, Marcus Browne and Marc Bola already at work.
Neil Warnock and coach Ronnie Jepson were among those watching on as the group were put through their paces on Thursday morning.
Take a look at photos in the gallery above.