To be honest, I've never embedded a subform on a subform but if I were to attempt, I'd apply the same syntax logic to refer to a second level subform as I would the first. So try
Code:
[Forms]![Main form name]![subform control name].[Form]![subform control name].[Form]![control name on subform]
(where [subform control name] is the name of the subform control on the parent form and not the subform itself) to drill down to a control on the second subform, or leave off the last control name reference to try drilling down to the second subform level. If you know how to use the Immediate window, you can try that syntax on a known control name to see if it returns the correct value when the form is open. That is, you'd type
?[Forms]![Main form name]![subform control name].[Form]![subform control name].[Form]![control name on subform], hit Enter, and see if Access reports resolves the reference. If not, you can try drilling down in stages using this method. Once you drill down to the needed level correctly, you should be able to affect a command on the form.
Even the incorrect use of the bang operator (!) versus period can cause this to fail, so know that I'm guessing here. The syntax for the normal situation, such as where a control on the subform is being referenced is
Code:
[Forms]![Main form name]![subform control name].[Form]![control name on subform]
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.