Wakefield hooker Davey, 23, has been on dual-registration terms with the Eagles in 2012, while former St Helens youngster Armstrong makes the move from Swinton.
McDonald, who has been with the club for five years, has signed a 12-month contract extension and back-rower Knowles has agreed another two years.
Head Coach Mark Aston said: “James (Davey) has been great since he joined us and I’m pleased Armstrong is on board with us too. They have Super League experience and will really add strength and depth to our squad for 2013.
“We want to continue bringing young, exciting, Super League talent to the club to develop and grow them as players even further so I’m highly delighted we have managed to secure the services of these two young lads for next season.
“I tried to sign Knowles at 19 years of age when he left Super League side Castleford but he’s been on a bit of a journey to Gateshead and Barrow.
“He’s a tough, hard, player. He broke his nose and possibly his hand on Thursday night (in the victory over Leigh) but didn’t go to Hospital, he’s a champion young man and I can’t speak highly enough of him.”
Armstrong scored the winning try for Leigh in the 2011 Northern Rail Cup final, and scored 13 tries in 24 appearances for Swinton in 2012.
Davey, who made 19 appearances for the Wildcats, has also had loan spells at Keighley and Featherstone.