The trio all spent time at Bramall Lane at the letter part of the 2007-08 season and all played a huge part in the Blades quest to reach the Play-Offs which they subsequently failed.
It's difficult to sort anything out until after the Premier League season has finished," Blackwell said. "Until such time, Premier League clubs, quite understandably, don't want to take decisions on which players they will be keeping and which players they are prepared to let go.
"I want to speak to the players involved and then to Wigan and Manchester United. And that's when we'll see what might happen."
It is likely that the Blades will offer Wigan Athletic a bid of £1,500,000 for winger David Cotterill and around £750,000 for Lee martin whilst defender John Halls sees his deal with Reading end on 31 June and will join on a free transfer.
All players would make great signings for the Blades and would pave enthusiasm in the Blades challenge for promotion next season.