Video: Barnsley release trio of players
Canadian international Jackson, signed on a short-term deal earlier this season, has struggled for opportunities at Oakwell, while loanees Wilkinson and Smith have failed to significantly impress and earn longer-term contracts - despite clauses in their deals stipulating that Barnsley had an option to purchase them permanently.
They have returned to parent clubs Bolton Wanderers and Swindon Town respectively.
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Hide AdMeanwhile, Reds head coach Johnson has revealed that the club hoping to keep Reece Wabara, who is contracted with the club until January 9, along with Aidan White and Lloyd Isgrove.
Johnson is due to talk to defender Wabara later today.
Johnson also wants to keep loan midfielder Ben Pearson for an extended spell, but admits that the Reds face competition with the future of the player, now back at Manchester United, uncertain.
Sam Winnall (shin) is a slight doubt for Saturday’s game with Millwall, but should be okay.