the carpet man wrote:
Whats the normal distance between posts using 5 x 3 posts and using 5
inch wide x 6ft featheredge, and 3 arris rails ??

3m maximum. Are you putting up a fence, or just asking?
At this length you ought to make sure that your rails are
of good quality, no large knots (especially near the middle
of the length), etc. Measure up and see what distances
"work". Consider fixing the boards in alternate bays in
different directions for reduced look-through.