I don't know what a 'standard 3-lane highway' is. But a 2+1 highway means one lane in each direction, as well as a middle lane that changes direction at certain points.
The advantage of these is that rather than forcing drivers to overtake into opposing traffic, they can wait until their direction gets the overtaking lane. These sort of roads are very common in Denmark. And I've seen a few of them in Germany as well.
(This is not to be confused with 2+1 highways where the middle lane change direction depending on the time of day, these have been phased out. There are usually clear markings on 2+1 highways when the direction change.)
A three lane highway is similar to a UK motorway. A road has six lanes, three (plus an emergency "hard shoulder") for each direction seperated by a central crash barrier.
The advantage of these is that rather than forcing drivers to overtake into opposing traffic, they can wait until their direction gets the overtaking lane. These sort of roads are very common in Denmark. And I've seen a few of them in Germany as well.
(This is not to be confused with 2+1 highways where the middle lane change direction depending on the time of day, these have been phased out. There are usually clear markings on 2+1 highways when the direction change.)