If record an acuity as 6/6, this means that the acuity was measured at a distance of 6 meters. Clearly not possible in a three meter room without a mirror. If you record it as 3/3 that will be correct, but confusing for many people. I prefer to use decimal notation for 3 meter rooms as this is correct and does not imply that the test was done at 6 meters.
If someone reads 1.0 and is plano in a three meter room, they will probably be -0.25 and read 0.63 to 0.75 in a six meter room.