Actually, Edgar, I had a similar situation with that same antenna a
number of years ago. Every now and again I noticed that channel 3 was
bad, but other times it was fine. I tried turning off and on various
AC and DC circuits on the boat. The two usual suspects - the battery
charger and the reefer - had no affect when either on or off. I also
had a small AC fan running, but it also had no affect. But when I
killed the AC circuit for the back half of the saloon, the problem
cleared right up.
In that circuit was my VCR. It was fed through a two-way pushbutton
switch to select between the antenna and the VCR. With the VCR turned
on, it was sending out a modulated signal on channel 3. Apparently it
was strong enough to get through the two-way switch and into the
antenna! Once I turned the VCR off, no problems.
Hope yours is that simple.

Dave