The left shifter controls the front gearing and the right shifter controls the rear gearing.
The rear has seven cogs that range from 14 to 34 teeth, specifically 14, 16, 18, 20, 22, 24 and 34. Note that these cogs are all evenly spaced except that there is a big jump between the sixth and seventh cog. The seventh cog provides a very low gear for the steepest hills.
The front has three rings that have 28, 38 and 48 teeth.
The difficult of pedaling is a function of the front teeth divided by the rear teeth. The lowest gear you have (for the biggest hills), in the small front ring and the large rear cog, has a ratio of 0.82. That means that the wheels rotate 82 times for every time you turn the cranks 100 times. The highest gear you have (for downhills) is 3.43, which means that the wheels rotate 343 times for every 100 cranks.
You can continue to do what you're doing if that provides sufficient gearing for you. But if you need further help on steep hills, you can shift that left shifter down to "1" and if you want to go faster on downhills or flats, you can shift that left shifter up to "3".
If you do all the divisions (e.g., in an Excel spreadsheet), you can see that there is some practical overlap in your 21 gear options. For example, if you put the left shifter in "1" and the right shifter in "7", it's the same gear ratio as if you put the left shifter in "3" and the right shifter in "2". Both of these produce two rotations of the wheel for every rotation of the crank.
As you get more experience, you can start to pay attention to the "chain line". The chain runs most efficiently and lasts the longest if your chain is parallel to the bike and not at sharp angles. So that means that you should in general use the "1" left setting with the low numbers on the right shifter, the "2" left setting with the middle numbers on the right shifter, and the "3" left setting with the high numbers on the right shifter. This is just a general guideline and you don't need to be obsessive about it.
To answer your specific question, to go from the lowest gear to the highest gear (not that you'd ever want to), the following are the settings for the left shifter and the right shifter respectively:
3,2 (same as 1,7)