The FKSM is a bit confusing. I re-read it after you pointed that out and it appears that for some reason the (x3) note is mentioned twice for the M3 CFVs in the Battalion Scout Platoon. The Scout Platoon should have three sections, each with one M3 CFV and one HMMWV (or some other combination of the 6 vehicles). This would make more sense and make the number of M3 CFVs in the Scout Platoon equal to that in a Cavalry Platoon.
You can read it yourself
here and make your own determination. What is also clear is that on page A-15 it says above the graphics "(X3) SCOUT SECTION," while it also shows another "X3" under the M3 CFV graphic. Looking at other entries, however, it appears that the number under the vehicle is supposed to be a total for all the formations of the type indicated.