I have a background for my app in resolutions 720x1280 pixels, 1080x1920 pixels and 1440x2560 pixels.
In which folders (mdpi, hdpi, xhdpi and xxhdpi) should I put each background?
Please read the Android Documentation regarding screen sizes.
From a base image size, there is a 3:4:6:8:12:16 scaling ratio in drawable size by DPI.
LDPI - 0.75x MDPI - Original size // means 1.0x here HDPI - 1.5x XHDPI - 2.0x XXHDPI - 3x XXXHDPI - 4.0x
For example, 100x100px image on a MDPI will be the same size of a 200x200px on a XHDPI screen.