Syntax Error - Generate Macro

I'm trying to build up a string to use as a file name but I'm having trouble getting the macro to generate correctly
My generate macro expression is
str('Var1') + "_" + str('Var2')  +" _" + str('Var3') +".txt"
where Var1,2,3 are attributes from the exampleset

what I'd like to end up with is a macro value like
Team2_Results_Summer.txt
Team2_Predict_Summer.txt
Team9_Results_Spring.txt

but all I get is a Syntax error even when the function express says the "Expression is syntactically correct" and the process won't continue after the Generate Macro operator