So how is that you are using the Classes A, B, and C?
Looks like you have objects of all the three classes in your problem, is it so? If yes, please review your design again.

Normally you would only need Objects (instance) of B and C (as Class A is already included in both B and C)

One important thing to note is the only objects can send signals, classes cannot send. (I agree that "class" and "object" are used almost equally in this forum replies, but it looks like from your question, you have a problem to differentiate between them)

You said Class A is base class, and Class B & Class C are derived (I assume derived from Class A). This is not multiple inheritance, this pain single class inheritance.

When you say Class A has QLineEdit and QPushButtons, then it is implies that Class B and Class C already have a QLineEdit & QPushButtons in them.

Please consider re-framing your question

http://www.cplusplus.com/doc/tutorial/inheritance/