Since each team only plays once with the others teams then the number of games is $\frac{(8)( 7)}{2}=28$. We must note that if the a team wins the other loses so if we have $k$ wins in a championship we must have $k$ losses in the championship if all the games were played so the number of wins is $28-12=16$. Now the total points of the championship is the sum of the points gained by the all the wins and draws, but we must noticed that if a team draws the other of course draws too so in a game we will have $2$ points per draw. So the total points in the championship is $(2)(12)+(3)(16)=72$. Since each team in first place has $15$ points then the total points in first place is $(4) (15)=60$ then in second place we have a total of $72-60=12$ points. Finally $N=\frac{12}{4}=3$ and we're done.