class Counter:
    def __init__(self, low, high, step):
        self.current = low
        self.high = high
        self.step = step

    def __iter__(self):
        return self

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            result = self.current
            self.current += self.step
            return result


for c in Counter(3, 9, 2):
    print(c)
# printed 3, 4, 7, 9