These are obviously the same girls photographed over a span of years. The only indication as to who they might be is some scrawled writing on the back of one of the pictures. It looks like it might be: Jenecal Belarag, but that may be a note that has nothing to do with the identity of the girls.
In the second photo they have reversed places, but the pose is otherwise very similar.
In the third photo they have switched places again and have both grown their hair long.
The stamp boxes on the backs of the cards provide a fairly broad range of the date, with the first and the third card printed between 1910 and 1930. Cards like the second one were also printed through the 1920s.
Here are the backs in the same order.