Increase time for animation

I have animation clip from 3 frame. I need to stretch this animation for 20 frame. I add some frame but animation dosen’t play in this frame. In the result animation is interrupted.
How to correctly increase the animation time?

Not sure this is what you are wanting, but if you want the whole animation to last longer I would go for the “rate scale” Setting on the left of the animation window under asset details tab and the Animation section. I would change that to a lesser value from 1.0 to .5 for 50 percent longer for example.