Going down hill is a little easier than going up. You still have the risk of blowing a hole in the pipe. With 3" pipe I think going downhill would be best at first if you have never done a pipe procedure. Just watch your heat I would run my amps between 70 and 80 or just what feels good for you.Go fast and stick that rod in there remember to keep your rod angle right at all times. I have done a little down hill with 6" pipe and I find it easier than 3" or 4" so give it a shot and tell us how you did. Good Luck.

First find out what the code calls for. If your code is 1104 you can go downhill, if it is under D1.1 you can only go uphill. In ither case you do need to watch your key hole, and the penetration on ither land. If you blow out your key hole try picking up your speed or lower your amps a little. Remember downhill you drag the electrode and uphill you whip and pause (think of fly fishing for electrode movement).
To get the key hole started in ithercase, you can long arc a little at first to get it started. I would recomend grinding your tacks as thin as possible and feather the edges, if your root is going well this will allow you to burn right through the tack without any defects. Also make sure your lands have a 1/8 inch face (the thickness of a nickle).
Good luck