I'm no expert, but I've had two classes in western equitation, and I was taught to line up my shoulders, hips, and heels. I tend to lean forward, not back, like a jockey. That's probably better than back. I try to sit on my seatbones, and tailbone. That's three point contact. I feel a lot better in the saddle when I can feel my seatbones. If I lost a few pounds, I'd probably feel my tailbone, too.
I dont' think your tailbone should ever be in contact with the saddle, at least not intentionally. The so-called "three point contact" is the seat bones and the pubic bone. This is more a dressage type seat than what western riders might use.
One thing to think about when loping is think as if you are a tree that grows on a hillside; the tree is always upright. If the slope is steep, the tree's angle will be tight, in order for it to be perfetly vertical. But, in a lope, there isn't THAT much downhill part, so you don't need to try so hard to create a downward position to maintain true verticality.
Think "down" on each beat of the canter wher the horse lands on its' leading leg. So, one, two , three of the canter rythm becomes "one, two, down". On that down beat, think aobut your heels doing DOWN into the stirrup, past the stirrup toward the ground. Your pubic bone goes DOWN , following the saddle downward, but your upper body stays vertical, no matter what.