I think you need to make a slight update to the if statements. Assuming the values fs1, fs2, and tw1 are text, they should have quotes so that they are not read as variables. I think it should be something like the following:
if(${locationID}='FS1','fs1',if(${locationID}='FS2','fs2',if(${locationID}='TW1','tw1','')))