Is it possible to tune a hub gear? I have a city bicycle with an internally geared hub. The problem with it is that all the 3 gears are too small, and I have a few hills around.

Is it possible to tune the gears so that they larger?
This is done by changing the primary gear ratio with your chainring and, if possible, the cog on the hub. 

If you make a large change or if the your existing chain is sufficiently worn, you'll likely need to get a new chain as well.

As mentioned in the comments, you need to be careful about making the gearing too small (easy to pedal), as you might end up exceeding the max torque for your hub. It sounds like, however, that you'd like the gear to be larger, not smaller.