I'm pretty positive you're screwed
I tried a similar thing with a GF2 and an ATI AIW tv tuner pci card a while ago. YOu cannot use the TV tuner functions unless the ATI pci card is the primary display, meaning in your bios the ATI MUST be the primary display and not AGP. I remember reading about this on the ATI site somewhere when I was trying to do this.
It said something along the lines that if using an ATI pci tv tuner card in a dual monitor setup, that it must be set as the primary display in bios. What that will mean though, if you are a gamer, which I'm sure you are with a gf3, your games will only play on whatever card is your primary display. In this situation, that would be the ATI - which is something you obviously DON'T want. YOu can't drag games or force them to open on another monitor or videocard.
And if you leave it setup the normal way. With AGP set as primary display, and the Radeon as your secondary card. The TV tuver functions will not work. As I said, the ATI card needs to be the primary display in order for tv functions to work.
I took my ATI card back because I did not see the benefit of having TV display if my games had to be played on a pci video card.