Road tyres on mountain bike
It's totally fine to use smooth tires on a mountain bike, if you use it as you describe. You will find it even more comfortable, just be a bit more cautious whenever it rains.

As @MikeBaranczak points, be careful with the size, 27, 29 are nowadays quite common. BUT ALSO have a look: 


how fat they are and if they fit with your actual wheel. 
valve type, again check it fits with your actual wheel.


Best and easiest way is to check what you have now and order the same size, valve and thickness but in flat.

About having to change them each 3 months, I would say is not normal (of course depends on km) but the first new-bike tires are usually a bit soft/not-good-quiality and they get flat very fast... maybe if you don't change them in 3 more months you have them already flat and you don't need to change them :)