United have signed forward Alex Nicholls, 29, on loan from Barnet for the rest of the season.
Carlisle midfielder Joe McKee is also coming to Scotland on loan for the rest of the season, with the 24-year-old joining Falkirk, subject to clearance.
And the Buddies have signed defenders Adam Eckersley, 31, and Harry Davis, 25.
Former Hearts and Hibernian defender Eckersley joins the Paisley side for the rest of the season after leaving Canadian club Edmonton, while Crewe Alexandra's Davis is on loan at St Mirren until the summer.
Elsewhere, Raith Rovers have signed defender David Syme following his release by Partick Thistle while Declan McDaid, who has also left the Jags, has joined Ayr United.
Meanwhile, forwards Tope Obadeyi and Justin Johnson have left Dundee United by mutual consent.
Of McKee's signing, Bairns manager Peter Houston said: "Joe is a target I have had in mind for some time and we have worked extremely hard throughout the day to get this one over the line.
"I believe I am adding both strength and quality to the squad with the signing of Joe having watched him in various matches over recent years."
Scottish Championship sides Dundee United, Falkirk and St Mirren have all made deadline day signings.