This is very general, but, with 3 cogs on the front, you should probably be in the 2cnd ring for your flat to semi-flat roads.
On downhill portions the 3rd ring (largest) on the front and for uphill portions use the first cog (smallest) on the front.
This should give you the best chance at maintaining a similar cadence in each circumstance, choosing between the available rear cassette rings to adjust within each of those ranges.
Don't worry about too much about the gear numbers, just think of it as easier vs. harder or as Craig mentioned, higher vs. lower.