The approaches above are perfect, but it's a pure ratio question, so you can start with a number if you prefer. The best number to work with is some multiple of your denominators, so you can start with 12 here. Then we have the following sequence of changes:

12 ----( -1/3 of 12, so -4)----> 8 ----(-1/2 of 8, so -4)----> 4 ----(-3/4 of 4, so -3)----> 1

so 1/12 of the lawn is not mowed.

One advantage of the approach above is that it allows you to answer almost any question quickly, without needing to think about what you would have to do algebraically. If the question asks "what was the ratio of the amount mowed on the first day to the amount mowed on the third" we can see right away that 4 units were mowed on the first day and 3 units were mowed on the third day, so the answer is 4 to 3.