Need helps with checkboxes

Hi!
I'm trying to create a GUI which contains several checkboxes, (exp: 3 checkboxes with tags: g1, g2 and g3). My initial string g = [g1 g2 g3], i want to remove g1, g2, g3 from initial string 'g' in cases they are not checked and update 'g' if these checkboxes are checked. Are there any ways to do this in just a callback function? As i'm trying now using separate callback function for each checkbox
Thank you for your helps!