Unfortunately, an amplifier will not correct your indoors reception problems. At only 3 miles from your local towers at 100' above the ground, weak signal strength is not your problem. If anything, you may have too much signal power complicated by 1) that building in your way, 2) multi-path, and 3) your own building's construction.
Go to
http://www.tvfool.com/?option=com_wrapper&Itemid=90 and enter your data using 100' as your height. Post the resulting link back here. That will give us a clear idea of what signals are forecasted for your location.