It's close, but not quite. The middle ring of a triple sits about 1.5mm further outboard than the centerline between the two rings of a double. (45mm triple chainline vs. 43.5mm double: http://sheldonbrown.com/chainline.html)

If using the middle ring mounting position of a triple crank for a single ring front, ideally, you'd use a 3mm shorter bottom bracket than intended for that crank. Of course, it's such a small difference that you'd likely not even notice.

For the OP, a MTB triple chainline is 47.5mm (possibly 50mm if on an oversize tubed frame). A double outer chainring on a road bike should be at 46mm so again you'd want to move the crank in by 1.5mm (or 3.5mm if oversize) if using only the two inner positions.