I am adding on to a small hunting cabin. The existing electrical panel is essentially a sub with six 20 amp breakers, three active and three spare. Leading into the panel I have one 12/2 wire and one 12/3 wire which were placed there for future use. My question: If I need more than the three 20 amp circuits in the addition, can I use the 12/3 wire to establish another sub panel in the addition. I am unable to run additional wire into the main panel.
Am I stuck with the three 20 amp circuits? If I run a circuit off a double pole breaker into a new sub panel using my 12/3 wire, how many circuits can I have and how large can the new circuits be?

A 12-3 will only give two 20 amp circuits so a subpanel is not a solution. Check to see if the 6 circuit panel can take half size or tandem breaker, can get two poles in one position.

Or change the 6 circuit panel to a larger one is best.