[ADTech]

It would seem (to me) that it would be HIGHLY unlikely that LTE frequencies would have any effect on the reception of a VHF 3 signal. In any event and assuming that you're buying and installing the CM3201 filter, those have > 40 dB rejection at about 730 MHz, the start of the most commonly used LTE blocks. A 700 MHz LTE signal, assuming ATT or VZN, isn't going to have signal strong enough at 3/4 of a mile to make it through even ONE of those filters, much less four or five of them.

I'd suspect something far less exotic, like a local lighting product or other electrical device in or very near the house is causing the interference.

Getting the antenna out of the attic and up into the air would likely do wonders.

Unfortunately, your DigiAir meter isn't going to be able to do much for you since you really need to be able to see the spectrum with a much narrower bandwidth around channel 3. It appears to only allow a spectrum display of its full channel span.