Accessing a nested MC from within a dynamic MC

Hi everyone. I’m a long-time user of AS2, and I’m just starting to get the hang of AS3. One barrier I’ve come to is the following:

I attach a movieclip named MC1 to the stage using addChild, and this works fine. I can control MC1 without issue.
However, there are 3 movieclips within MC1; one on frame 1, one on frame 2, and one on frame 3. They’ve each been given unique instance names - MC1a,** MC1b**, and MC1c. And for some reason I cannot access them in the standard way of MC1.MC1a …etc. I am given the following error:

Cannot access a property or method of a null object reference.

I’ve searched the forum and Google, but the issues I’ve found that were resolved are different from my specific problem, or I was simply unable to apply the solutions successfuly to my own problem due to my lack of experience. I’d greatly appreciate any input or help on this.

Cheers

Brendan