myLists = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

def to_one_list(myLists):
    myList = []
    for _list in myLists:
        for l in _list:
            myList.append(l)

    return myList

print(to_one_list(myLists = myLists))