You'll be fine. Actually you could have gotten a single 1.8amp 12vDC converter, since the 3 fans each draw .6amps the converter needs to put out amps equal to the total draw.
Actually it doesnt have to be equal, greater then is also fine.
And for each volt over that requirement goes down
EG: 14V would be enough for the 3 fans at 1A and be ok.
There is a way around it though.
If you wire them in seriel, rather then paralel. the added "force" changes it
(.6 + .6 +.6 ) / 2 = 1.2A
Youll get away with it fine at that rating
they will spin just a bit slower.
and since they are in Serial, the added resistance to the line will prevent burnout.