Not likely to happen. The components in phones are so tightly packed in, and the entire body is built around exact specifications. LTE radios are larger than 3G radios, so it would probably require modifying the entire enclosure of the phone to add one, unless they make the phone extra fat from the beginning. I doubt they'd do that. On a tablet, such as the Xoom, there's a lot more space to work with, so it's easier to be able to upgrade the radio (although it must not be too easy since Motorola still hasn't done it).
Although there haven't been any rumors or indications that this is the case, if they want a LTE Droid 3, it's more likely that they'd release an updated version of the phone later on. They did something similar by pulling the Droid 2 a few months after it was quietly released and replaced it with the Droid 2 Global, so it's not unprecedented.