I have imported a layered photoshop file as a composition. When I open the composition it is only 3 frames. I try to extend the length but I can't.
I go to composition settings and see that composition length is at 3 frames. I change it to 5 seconds and reimport the file.
It is still a 3 frame composition.
Once you change the comp length, you also have to change the out points of the layers in the comp.
You have inadvertently checked the box that says Photoshop sequence. This is a common and easy to make mistakes so don't feel bad. Import the file as a comp again but make sure the box is unchecked. Then you will be able to change the length of you composition of the composition settings.