Dundee United's Charlie Miller, John O'Neil of Hibs and Dundee's Gavin Rae have all been included after impressing with their respective clubs this season.
Celtic keeper Rab Douglas is also in the squad, although he has been in previous squads on several occasions.
Rae is the only one of the outfield three to have been involved in the squad before - he was called in to the squad to face San Marino after Barry Ferguson's withdrawal, but did not play.
Scott Booth, who plays for Twente Enschede in Holland, has been recalled to the squad after a lengthy absence.
Defenders Colin Hendry and Matt Elliott miss out - Elliott has an injury while Hendry has club commitments with Bolton who are chasing promotion from the English First Division.
He is also awaiting a decision from Fifa over his elbow on San Marino's Nicola Albani.
Brown explained the reasons for the changes to the squad: "Scott Booth comes in on merit. My colleagues Alex McLeish and Andy Watson looked at him last week and the reports were very favourable indeed.
"Charlie Miller has always been a talented player and now I think his club manager Alex Smith has got the best out of him and this is a new challenge for him.
"Regarding John O'Neil, the weekend helped him and I looked at his recent form and thought he deserved a call-up, and also this is an opportunity to look at Gavin who has been playing well for Dundee this season."
Squad:
Goalkeepers: Robert Douglas, Neil Sullivan
Defenders: Tom Boyd, Christian Dailly, Calum Davidson, Jackie McNamara, Steven Pressley, David Weir
Midfielders: Colin Cameron, Barry Ferguson, Scot Gemmill, Paul Lambert, Charlie Miller, John O'Neil, Gavin Rae
Forwards: Scott Booth, Paul Dickov, Billy Dodds, Don Hutchison, Allan Johnston.
Under-21 squad:
Esson (Aberdeen), Langfield (Dundee), Stewart (Kilmarnock), G Caldwell (Newcastle), S Caldwell (Newcastle), Canero (Kilmarnock), Hammell (Motherwell), McGroarty (Dunfermline), McGuire (Aberdeen), Cummings (Chelsea), Hughes (Bournemouth), McCunnie (Dundee Utd), Murray (Hibernian), Severin (Hearts), Smith (Celtic), Stewart (Manchester Utd), Burchill (Celtic), McManus (Hibernian), Miller (Rangers), Parker (St Johnstone) Young (Aberdeen).