Thomas, a loose head, has been part of the matchday squad on several occasions this season and made his first start in the LV= Cup game against Worcester Warriors in January.
Knight, who started at tight head for England U20s throughout this year's RBS Six Nations, also made his first team bow as a replacement in the Worcester game.
Harden, 24, joined Gloucester last summer from Tynedale having initially come through the Newcastle Falcons Academy.
The strapping tight head has spent time with Moseley in the Championship this season but has also made five first team appearances for Gloucester.
"The front row is always a key area for any club to get right, and in Rupert, Shaun and Yann we've got three of the best young props in the country to come through and support the 1st XV," said Head Coach Bryan Redpath.