Gillingham have signed Bradley Garmston on a three-year contract, while Ben Williamson has agreed a two-year deal after rejecting new terms at Port Vale.
And goalkeeper Glenn Morris, 31, has signed a new one-year contract.
Left-back Garmston, 21, impressed for the Gills last season while on loan from Premier League side West Bromwich Albion, who recently released him.
Striker Williamson, 26, scored 28 goals in 132 appearances during his time at fellow League One club Vale.
"I was at Port Vale for four years and sometimes it's good for a change, and I'm moving back towards London which is a bonus," he told the club website.
Manager Justin Edinburgh said: "He [Williamson] will prove a handful for any defence in this division
"Bradley was impressive during his loan spell with us and I have no doubt he will be a valuable asset to us now he's with us permanently."