SELECT (SELECT SUM(Cw.Weightage) AS CompanyWeight FROM CompanyWeight CW WHERE CW.CompanyID IN (@CompID )) + (SELECT SUM(LW.LocWeight) AS LocWeight FROM LocationWeight LW WHERE LW.CompanyID IN (@CompID ))+ (SELECT SUM(CPW.Weight) AS ContWeight FROM ContactPersonWeight CPW WHERE CPW.CompanyID IN ( @CompID))
try this!
[Edit-karthik]: added in code block.