Do you need the list to show on the form itself, during that process step?
If you're able to add an intermediate step, there is the option to set the default value of your field to <list of all checked>. You can also assign metadata in the same way in a Save to Repository task. This would add the commas and cover any potential combination of options, without writing a huge IF statement.
Editing to add:
If you're wanting to write it out as an IF statement, you can do so using something like this:
=CONCATENATE(IF(Checkbox.Choice_1,"Choice 1, ",""),IF(Checkbox.Choice_2,"Choice 2, ",""), IF(Checkbox.Choice_3, "Choice 3, ",""))
Editing again (sorry!):
I realized that left a ", " at the end, so here's an updated formula to remove that:
=LEFT(CONCATENATE(IF(Checkbox.Choice_1,"Choice 1, ",""),IF(Checkbox.Choice_2,"Choice 2, ",""), IF(Checkbox.Choice_3, "Choice 3, ","")),LEN(CONCATENATE(IF(Checkbox.Choice_1,"Choice 1, ",""),IF(Checkbox.Choice_2,"Choice 2, ",""), IF(Checkbox.Choice_3, "Choice 3, ","")))-2)