Any XML attributes prefixed with tools are removed when the app is compiled and are only rendered by Android Studio layout editor.
You need to properly set constraints in your ConstraintLayout, not use absolute positioning.
Or you may instead use RelativeLayout, LinearLayout, etc.
The problem is that your views are not properly constrained (basically they aren't linked to anything so when you run your app it doesn't know where to place them and just defaults to the upper left).
Solution 1 - Right Click on you Layout and select "Convert View" Option. then select "LinearLayout" Option. it will show all three buttons in the output.
Otherwise read about Constraints
HERE, once you use the guidelines, your buttons will display correctly.
EDIT: As was rightly pointed out by David Crow, the width is set to the same as their parent causing buttons to overlap and lay on top of each other.