3 wood

i can hit my 3wood relatively well off the tee and out the rough but for some reason i can not hit it off the fairwaay

my question is

how come i can hit my wood alright out the rough but not the fairway?

Alex what happens to the ball from the fairway?

It's most likely you technique Alex I suspect you pick the club up too steeply and come down almost on top of or just catching the bottom of the club face. Take the club away low and slow and sweep it off the fairway and do't swing to aggressively. A 3 wood is very easy to hit off the fairway with this technique.Good luck.

I strike my 3 wood quite well from the fairway, averaging 230-240 yards.  A low ball flight is normal as the club is not designed to get the ball up in the air (mine only has 15* of loft). 

Fairway woods are played differently off the turf to off the tee, and differently again to irons.  Irons are designed to strike down on the ball, squeezing it against the turf to get it up in the air and give it backspin.  This means you should hit them on the downswing so they are played further nack in the stance.  Woods (and irons) from the tee should be struck on the upswing, getting the ball airborne and minimising the backspin to get greater distance.  This means they are played further forwards in the stance.  When playing a fairway wood off the turf, the aim is to sweep the ball, striking the ball as you transition from the bottom of the arc to the the upswing to get the ball airborne.  This means the ball should be positioned further forward in your stance than your irons, but further back than playing it off the tee.  I suspect you are playing the ball out of the rough OK because the grass is teeing it up slightly and you are playing it like a tee shot without realising it. 

It's difficult to be sure without seeing you swing, but I suggest you have the ball in the wrong position in your stance when playing off the fairway.  Try taking a practice swing and seeing where the club skims the floor (there shouldn't be a divot).  Then place a ball an inch forward of this position and take the same swing.  You should see better results (assuming you can replicate the same swing!). 

Can't guarantee that this will solve the problem, but it's a pretty good place to start.  The pro that taught me how to play said to me playing woods from the fairway is the most difficult shot in golf so don't get too downhearted if it takes a little time to get it right.