If I take a flight from Boston to London via Iceland, do I go through security just in Boston or will there be one in Iceland as well? Also the layover is 3 hours, I assume this will be adequate?
Should be more than adequate. Whether you go through a second security check will depend completely on the airport layout.
Frank is right. If you are just connecting within the terminal, chances are you are just going from one end of the airport to another. Go online to the iceland airport and get an idea of their layout.
Chances are you should be just fine after Boston.