I can think of two variables:

1) How busy a passport control will be at the time of your arrival?
2) Will an agent at the passport control give you a fuss when see you are departing in three hours?

The first situation, people can only speculate from previous experiences but will not know for sure until you actually arrive at the passport control.

I heard of people able to do stuff like that so I kind of think the situation number two is something less to worry compare with situation number one, but you never know.

Even if you get thought without any problem, you may have about an hour for the shopping. If you have something specific you are looking to purchase, then people here may able to provide information regarding availability at airport shops.

As far as passport control and security going back to the gate area, it is usually minimum wait unless it is major holidays.