Thanks Richard for trying to help.  I have not yet been successful in getting 3 nested combo boxes to work on a continuous form.  Here's what I've tried:
1) I tried the code you suggested, substituting my form/subform/field names:
Forms!FormName!SubformName.Form!FieldName

If ComboBox A is based on straightforward data with no criteria (ie States table), and ComboBox B (cities) is based on ComboBox A(states), when I enter data in Row 2 it doesn’t seem to know that I’m now in Row 2 on the subform.  I get the same error as before "Enter Parameter Value" box with Forms!frmName!sfrmName.Form!cboField

In the Parameter Box that appears for ComboBox B, if I enter the ID value that is the IDcriteria for the ComboBox A in the underlying query ... such as 1, 2, 3 etc. ... then the proper choices appear in ComboBox B.  This carries through with ComboBox C which is built on ComboBox B.  

2) I also went thru your Access Lesson 310.  In TrackingPayments1, I found a reference to getting values on a main form that are based on a field in a subform.  I've done this before.  However, my current dilemma is different.  My combo boxes are all on the subform, and they reference SQL code or an actual query. The combo boxes or underlying data don't have anything to do with the main form.  

I’d be really grateful if you can figure this out.  Thanks so much for your time!