The MAC addresses being similar on the first two compared to the third does not matter, as long as all the addresses appear correctly in the configuration utility.
Plug in all three where you will use them. If you attach a computer to the new adapter and run the configuration utility, do all the devices show up and have an identical network name? If not, go and attach the same computer to one of the two original adapters and check the information on those two and note the settings, then re-attach to the third and check its configuration settings. I would check the MAC address of the third unit to be sure that it is the same on the adapter label.
Are all three units running the same firmware?