Well, if 1 mile = 1.6 kilometers and he's traveling at 18mph, then do 18 x 1.6 which = 28.8.
So, this is his current speed in kph
3 x 28.8 = 86.4
They will have traveled 86.4 kilometers
(I haven't worked with this kind of math in a while so correct me if I'm wrong!)