A television tuned to channel 3 (60 MHz - 66 MHz) experiences interference when you are transmitting on the 15-metre band. Other channels are not affected. What is the most likely cause?

The 15-metre amateur band is located at approximately 21 MHz. If a transmitter on this band radiates a third harmonic, the frequency will be 3 times 21 MHz, which is 63 MHz. This falls squarely into the 60-66 MHz range of TV Channel 3, causing interference specifically to that channel. If the TV were suffering from front-end overload, it would likely experience interference across multiple or all channels, not just one. While high-pass filters can prevent fundamental overload, they cannot block harmonics that fall exactly onto the TV receiver's tuned frequency.