I'm sorry but most of the answers here are incorrect. The original observation made by the person asking the question is correct. In 3 dimensional space, only three unique co-ordinates are needed to describe an objects exact location relative to a known point of origin.
A good real-life example of this, which proves that point - is that GPS devices are designed to track your location with a minimum of three satellite signals in range. GPS co-ordinates do use more satellites than that to improve accuracy, but that is not because having more reference co-ordinates makes them more accurate, it is because having more RADIO signals reduces the impact of noise and other interference on the that distorts one of the signals it is tracking.
Imagine your assistant is sitting in the backseat on the passenger side of your car, and you need to tell him the exact location of a cup of coffee you left on your dash board. You could use the following reference co-ordinates:
Front Bumper (X axis coordinate)
Drivers side-view mirror (Y axis coordinate)
Windshield Wipers (Z axis coordinate)
Now, from the passenger side back seat if you were to move your hand towards all three of those object, without moving passed any of them, you will arrive at a location that sits in front of you, at the height of the windshield wipers, in the direction of the drivers seat, and that would be the dashboard of the vehicle in the corner closest to the drivers window. Adding 3 more reference points would not make it any more accurate, because it doesn't matter how many directions you measure from, as long as you have enough reference points to choose from.
However, I think that 3 million possible address combinations and 38 reference points is accurate enough when you are talking about galactic travel. If more accurate was needed, they would have to use symbols on the gates, but the addresses would still only need 3 symbols and a point of origin to work!
For those of you who believe the other answers that argue that 6 points plus a point of reference is logical; I can understand why you might come to that conclusion but you are over thinking it... and you are wrong. If you were to eliminate the point of origin from the equation, then you would need to 6 coordinates, just because you would need 3 coordinates for each point (one set for your point of origin and one set for your destination). Hollywood obviously did not understand the math, and misinterpreted how coordinates work in three dimension space.
Another way you can see how illogical that would be, would be to take a two dimensional map and mark two points... then figure out how to describe the position of one point relative to the other. According to Stargate logic, you would need one coordinate for your point of origin, and then four points for the destination (one point on each side of the destination forming a square, instead of the cube they used in the movie for 3 dimensional space). I promise, when you are done, you will feel pretty stupid. It won't take you more than a couple seconds to realize that finding four reference points to describe one location on a 2 Dimensional map is completely stupid.
And for those of you who are die-hard over thinkers, you do not need an agreed center or universal axis, or reference orientation to make this work using one coordinate per dimension, plus your point of origin; as long as you ensure that every destination has at least three reference points around it which are all further away from the destination than they are to the point of origin (I won't get into the math or explain that part in detail, but the reason is that you need to draw three separate vectors going all the way past the destination from three different angles, so that the distance between the object remains equal to the distance between the point of origin and the intersection of the three points).