You know how to shoot, so just concentrate on the basics. Find your natural point of aim, breath for each shot then fire on your natural respiratory pause. Press the trigger without dragging wood, and follow through. Use a front and rear rest to your advantage if its allowed, otherwise use the sling if the rifle has one.
Things that might upset the apple cart are wind and mirage. If you can arrange to settle the bet early in the morning, before the breeze picks up and before mirage becomes an issue, that would work to your advantage. The key is that you have to hit the target on demand with the conditions at the moment, and often doing something on demand is more difficult than doing it under ideal conditions. I would not be inclined to have someone else zero the rifle unless you have the opportunity to verify the zero without using one of your three rounds. Try to find out before hand what his load is and work out come-ups required for a 550 yard zero. At the shot, get back on the scope as quickly as you can in an attempt to see the hit, if you can see the hit, you'll know how to hold for the final two shots.
I hope you take his rifle, anyone who makes such a reckless bet deserves to loose it.
