To calculate 5 under 8 of 3 m, we need to divide 3 m by 8 and then subtract 5 from the quotient.
3 m ÷ 8 = 0.375 m
0.375 m - 5 = -4.625 m
However, since we are talking about a measurement, the negative sign does not make sense. So we can drop the negative sign and say that 5 under 8 of 3 m is 4.625 m.