3. prepare vector, A, and B as above and do a triple ligation reaction. If you have good vector and A and B DNA samples and properly cut ends, then triple ligation is quite easy. I'd recommend cloning into a vector with different antibiotic resistance than the ones containing A and B. You can reduce vector only background by using PCR to amplify your vector as well, and then digesting with DpnI to remove undigested template DNA. If you do this with enzymes that can be heat killed, you don't even need to purify your digestion products.