3000 feet in 12 miles is 250 feet per mile. That's a reasonable average grade, not an extreme, steep climb. You'll be fine with your triple. Enjoy the ride!

If it was over 400 feet per mile, then the whole climb is steep, and some sections of it would be very steep.

EDIT--those feet per mile numbers are for one-way climbs, not up and back down rides.

My heart rate monitor helps me on long climbs. I have a target heart rate that I know I can sustain for hours, and I mostly try to keep at that pace. Like the other posters said, do your own ride, don't try to match a fast climber.

My "training" for long climbs is to ride a fairly flat route for a couple of hours right at my target heart rate, and avoid coasting as much as possible. I just want practice in long, steady efforts. Other than that, I'll do fast (for me) group rides to get some high intensity riding.