In 1 meter there are 1,000 millimeter.
When you go from small (mm) to big (m) you always divide the small (mm) by what the equivalent of 1 big (m) but in terms of the small (mm)
^^^That's a bit confusing so I will work it out so it makes more sense...
^ this is the equivalent to 1 meter (the big in this case)
3 m = 3,000 mm
Hope this helped! Let me know if there is anything else that I can do!