Luidog,
I think 3 times the screen width is a little to far away IMO. I believe the many discussions I've read on the forum suggest anywhere from 1.5 to 2.0 times the width of your screen. IMO there is no set formula on this, just some rules of thumb, it really comes down to personal preference.
My Firehawk is 9.6 feet wide and my first row of seats are at 16.5 feet (approx 1.7 x screen width) , my second row is around 24 feet (approx. 2.5 x screen width). Some prefer the front row, some prefer the back row.
I agree with "tlight" about taking into consideration your room variables. I spent a lot of time determining screen height, seating placement, rear seating riser heights, etc. You don't want to be moving your head from side to side or constantly looking up to see the screen, and you don't want the front row to block the back rows view when the seats are in a reclined position (assuming you have a back row, many don't).
There are many threads on screen size and placement, as well as seating placement on this forum, do some searches and you will get what you need to make an informed decision.