A. Benadryl comes to mind. (Put down your letter-to-the-editor pens, everyone. I'm just joking.)
Once your baby equates her car seat with the loss of freedom and the loss of Mommy (she is still rear-facing, right?), there's not much you can do to change her mind. Fortunately, that's not the goal. The goal is to survive a three-hour drive.
To do that, you don't need to make her like her car seat. You just have to distract her enough so that she forgets that she's in it. The best distraction is sleep, of course. If your schedule and stamina permit, why not travel at night? Wait until your baby is asleep, then strap her in and sit in the next seat so that you can comfort her when she wakes up.
Speaking of comfort, does your daughter have a favorite blanket or stuffed animal? In the days before you go on the trip, make it scarce. That way, when it magically appears in the car, your baby will go for it. Giving her a new toy or one that's been at the bottom of the bin for a while may also keep her entertained.
Another good distraction is food, so you'll want to have that ready. Of course, if you're still nursing, you already do. Not that I advocate leaning over your baby and breastfeeding while traveling at 55 mph. I'm just saying it's an effective solution until you can get to a rest stop. I know this from experience, and so do more than a few truckers who have passed our minivan on the highway.
Keep in mind that your baby may simply refuse to be placated. In that case, just try and hold on, reminding yourself that she's fine, and that you're entitled to a shot of Benadryl or another liquid sedative when you arrive. Either that, or host the holiday at your house.