Had the same Ch3 harshness issue for a while
Then I found I could fatten things up by opening the CH Vol to 2pm, and balanced the EQ to suit the pups I was using. I was keeping it below 11am which didn't work well. To my ear, CH3 needs power amp grunt, not preamp fizz. As some suggest above, I also run with EL34 = C=s and darker preamp tubes
Still not as comfortable in CH3 than 1 or 2 where I have found killer sounds, but its getting closer
More tweaking required - it never stops with this amp
Then I found I could fatten things up by opening the CH Vol to 2pm, and balanced the EQ to suit the pups I was using. I was keeping it below 11am which didn't work well. To my ear, CH3 needs power amp grunt, not preamp fizz. As some suggest above, I also run with EL34 = C=s and darker preamp tubes
Still not as comfortable in CH3 than 1 or 2 where I have found killer sounds, but its getting closer
More tweaking required - it never stops with this amp