Go to Leonardo and he will have 3 codex's available for you.

Select one and buy it. Once the cinematic has ended go to the DNA and play sequence 4 memory 8 - which is called "an unexpected visitor".

Once the cycle starts again just skip it and you will be offered the final 2 more codex's.

Select one of the remaining 2 and follow the same routine. You will have all 3 Codex's within a couple of minutes.