dayyou said:
(ie, would you add the 3 mph or subtract it)?
I this last part is unneccessary and possibly confusing. In this problem it's all vector addition and simply applying the correct coordinate system (and therefore direction). A better example would be with a tailwind, or some description that doesn't include subtraction. In the end you reach the same answer, but if you can handle this without pure vector addition, then you've already mastered the concepts beyond needing help.
To the OP, your coordinate system will look like a compass. You will start in the middle. The plane is moving in a certain direction at a certain speed and the wind is blowing in a different direction at a certain speed. You can take both vectors into account separately and come up with a third vector that gives you the REAL speed/direction of the plane.
I'll change dayyou's example to be walking at 15 mph (more like sprinting), but with a 3 mph wind at your back. Both vectors are in the same direction, so it's as simple as adding them together (18 mph in the direction the person is walking). If it was a wind that was blowing directly from the side of the walking person, they would still have a forward speed of 15 mph, but would also be pushed to the side. Combining the two vectors would result in the actual speed/direction of the walker. The triangle that you need to form is those two vectors added together to make a third. In the example with the wind at the back, the third side of the triangle is simple laid over the other two legs (so there's no real triangle).
I don't think I can give anymore without totally answering it for you, so at least try to piece it together. Don't just play with the numbers, but draw a compass and try putting the vectors for the airplane and wind on it.