Three-laned roads are where there is a passing lane in one direction and vehicles travelling in the other direction are permitted to use it if there's nothing overtaking that's coming towards them.
The line between your lane and the passing lane and carriageway in the opposite direction will have a broken white line and a solid yellow line if the road layout means you have enough visibility to overtake. If there's a solid white line and a solid yellow line then you must not overtake, even if you think you can see that the way is clear.
Passing lanes are most frequently found on uphill stretches which means you will more than likely be overtaking downhill. Be careful as your speed will pick up much more quickly and it will take you longer to brake. Check out this article for more about driving downhill.
Be careful if there's a truck or other large vehicle coming towards you that might be concealing a vehicle that wants to overtake it.