You could replace I, II, III with 1, 2, and 3. That might be the easiest approach here.
Substitute ( CheckBoxField ; [III ; 3 ] ; [ II ; 2 ] ; [I ; 1 ] )
can convert the roman numeral format to standard format.
You can use this expression with Replace Field Contents to update the data in all your fields in one operation. You'd then need to update the value list used for your check box group to also use the new format or the boxes in this field will all appear to be unchecked.
You can also keep the data unchanged and use this expression for a calculation field and then perform your searches on the calculation field.
Thanks so much!!!