Hi:
You could use a different character to separate your items, like a % or something, then use translate to replace that with ', ':
translate(eval(eval(group2, 'concat(my:field3, "%, %")'), ".."), "%", "'")
for example. Of course, getting your opening ' will take another concatenation:
concat("'", translate(xdMath:Eval(xdMath:Eval(my:group1/my:group2, 'concat(my:field3, "%, %")'), ".."), "%", "'"))
The trick would be to use a character that the entries won't have -- not too bad if the field uses a drop down list, but a bit tougher if the user can enter whatever he or she likes....