If I understand your question correctly, you need to use the Hypergeometric distribution. This distribution is usually associated with urn models, i.e there are n balls in an urn, y are painted red, and you draw m balls from the urn. Then if X is the number of balls in your sample of m that are red, X has a hyper-geometric distribution.
For your specific example, let n_A, n_B and n_C denote the lengths of your three lists and let n_A_B denote the overlap between A and B. Then
n_A_B ~ HG(n_A, n_C, n_B)
To calculate a p-value, you could use this R command:
#Some example values
n_A = 100;n_B = 200; n_C = 500; n_A_B = 50
1-phyper(n_A_B, n_B, n_C-n_B, n_A)
Word of caution. Remember multiple testing, i.e. if you have lots of A and B lists, then you will need to adjust your p-values with a correction. For the example the FDR or Bonferroni corrections.