Alan Knill says Scunthorpe trialists Gavin Gunning, Kieran Djilali and Dominic Green need to do a "bit more" to earn contracts.
Blackburn defender Gunning and former Crystal Palace winger Djilali both started in the 2-0 pre-season win over Rotherham on Wednesday night with ex-Peterborough midfielder Green introduced as a second-half substitute.
Knill was pleased with the performances from the trio but they still need to shine if given the chance in Saturday's final warm-up clash against Barnsley.
"I thought they did OK," Knill told Sportinglife.com.
"Gavin Gunning at left-back did really well, Kieran started well but just tired a little bit towards the end and Dom came on and showed he's a good footballer.
"We just need a bit more from them.
"We'll make a decision after Saturday and then we can concentrate on what is the squad."
Knill was pleased with the Iron's win at the Don Valley Stadium after some disappointing results in their pre-season campaign.
"I'm delighted," he added.
"I thought we were excellent in the first half. I think the biggest thing was we stopped them from playing.
"It's been quite frustrating but two games to go before the season starts to put in a performance like that gives us all a lift."