What is an acceptable rate of climb (inches of elevation per feet of track) for 3-rail?
Are you running magnetraction?
If not, I hope you are not going too high or have a lot of room. A friend made an incline on his O gauge layout for running pre-war (no magnetraction) and his engines would not climb it. Also, be sure you do not have any curves at the bottom of the hill, as the trains will have a tendancy to derail.
It depends a lot on what you’re running; but I wouldn’t expect to get by with more than 4 percent.