For objects orbiting the sun there are some very handy natural units.
Orbital radius - AU (1 AU is the average sun earth distance)
Orbital period - Year (time for earth to orbit the sun)
Mass = mass of the sun
Then Kepler 3 is just T^2/ R^3 = 1
And if you do your working out this way the answer will pop out in units of earth year without any further conversion.