England midfielder Wilshere has been sidelined since June 2011 with ankle and knee problems, but he played 74 minutes as goals from Serge Gnarby and Chuba Akpom sealed Arsenal's 2-0 victory over Reading at Underhill.
Frimpong is nearing full fitness after suffering a serious knee injury on loan at Wolves, while full-back Sagna is recovering from a broken leg and they also improved their match sharpness.
Arsenal reserves coach Terry Burton is pleased with Wilshere's progress.
"He rode challenges better which is a big part of his game," Burton told Sky Sports News. "He likes to keep the ball close to defenders to put them out of the game and I think he did that better tonight [Monday].
"I think his final pass was good. For me it was a big improvement on his previous performance."
Burton admits that Wilshere's rehabilitation has been a gradual process, but he is pleased with his determination to return to fitness.
"It's only normal it does come in those stages, it doesn't happen overnight," he said. "He's worked very hard with the fitness people, he's been back in training in our group for a couple of sessions as well and he's been first class in those."
Clubs are allowed to field up to three overage players in such under-21 fixtures.