Write a function named flatten_lists that takes a nested list as input and returns a flattened list containing all the elements from the nested list.
nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
Explanation: The function flatten_lists uses list comprehension with nested loops to iterate over each sublist in the nested list and over each element in each sublist. It creates a new list by extracting all the elements from the nested list and returns the flattened list.