Can mass be expressed in grams kilogram's and milligrams?

Answer 1

It depends on the mass and context, but it would be expressed in just one of those units with decimals when necessary. You don't mix unit names

Examples: 3mg, 7g, 7.53g, 6.4kg, 8000kg.

Note 6.4kg not 6kg 400g, for although numerically right the latter breaks one of the points of decimal measurements.

if you're working with extremely tiny or extremely large objects then scientific notation may be appropriate:

3 X 10-11g (0.03 nanogrammes)

5.5 X 1020tonnes... which would equal of course, if rather absurdly, 5.5 X 1023kg.