They probably provide the same amount of juice as a mobo fan header would so 3 on each is should be totally fine.
I think the typical manufacturing variance tolerance for fans at max rpm is +/- 10%, so if the speed you are seeing is within that 10% spec, then it's nothing to worry about it.
At lower power levels the variance is also typically higher. For example my Noctua fans have a +/- 20% at their minimum rpm.
