3/10 is the same as "3 divided by 10"
3/100 is the same as "3 divided by 100"
When we divide a number "x" by another number "y", we repart the number x in a number y of times in smaller parts of equal size
For example, if we want to divide 6 apples between 2 people, we will have 6/2 = 3 apples for each one. Then, we will have 2 persons with 3 apples each other.
As greater is the number of people, smaller will be the number of apples for each one.
For example, if we want to divide the same 6 apples between 3 people, then we will have 6/2 = 2 apples for each person, and not 3 apples for each one anymore.
That is why 3/10 is greater than 3/100.
In a example, we can think like if we were trying to divide 3 kilograms of food between 10 people. In this case, each person would have 300 grams of food.
But if we were trying to divide the same 3 kilograms of food between 10 people, each one would only have 30 grams of food, which is smaller than in the case where we are dividing food between only 10 people.