Whether you can download extension for Radio or checkbox from Qlik branch.
Or if you use the standard listbox from Sense, you can create a dummy table with 3 values: 3 mth, 6 mth & 1 year.
So when you select, your expression will put in IF statement
If(GetFieldSelections(Time) = '3 mth', <exp1>, etc...)
Or you can use variable to make it easier