Hello, how do I implement this in Haskell please? and for numbers 1 & 2 don't use the !! operator

1) a function that returns the third last item in an input list. I cannot use the !! operator. Assume the list has at least three elements

2) a function to return a list containing every other element starting with the first element. The function should work regardless of size of list and may have even or odd length. Assume the list is not empty.

3)a function to return sum of all positive numbers in a list. The sum of numbers in an empty list should be 0.

Please explain how you implemented these functions. Thank you