Overload closely resembles a signal that's too weak. You'll see and hear nothing on the TV, and the signal-level meter (if the TV has one) will display a low and erratic reading. Those three stations averaging 15 miles out are probably pretty strong in your area, so a CM 7777 is likely to overload no matter what. It's not often recommended anywhere within 35 miles of a full-power transmitter.
You'll have a far better chance at those distant signals with:
1) A high gain, all-channel antenna, mounted as high up as you can safely get it. Seventy miles is deep-fringe territory, so look into a long-range antenna such as the Winegard HD-7698P. Please note that the higher the gain, the more directional the antenna. Practically speaking, this increases the chance that a rotor would be needed to capture all desired stations.
2) A Winegard HDP-269 pre-amp. They're known for resistance to overload, even amid extremely strong signals. They also sport enough gain for most homes, even with two or three TVs.