Calculate criterion values for each confidence level (within SDT)
There's actually a very simple answer to this question: calculate criterion of confidence level = 3 by counting as hits only those trials where the answer was correct (and the target was there), and confidence was equal or higher than 3. 

This is also what Rahnev et al did in the following paper:
Rahnev, D., Koizumi, A., McCurdy, L. Y., D’Esposito, M., &amp; Lau, H. (2015). Confidence Leak in Perceptual Decision Making. Psychological Science, 0956797615595037. http://doi.org/10.1177/0956797615595037