In a county where a road has been or shall be built under the provisions of an act entitled “An act to authorize the board of chosen freeholders of any of the several counties of this state to lay out, open, construct, improve and maintain a public road therein,” approved April seventh, one thousand eight hundred and eighty-eight (L.1888, c. 274, p. 397), or any act supplementary thereto or amendatory thereof, there shall be elected at large in the county three commissioners to be known as “boulevard commissioners of the county of ” , who shall hold office for three years from the first Monday in December after their election and until the election and qualification of their successors.