Upload 3 different featured image sizes to WordPress

I believe you are looking for add_image_size.

You need to put this into your functions.php to register new image sizes. WP will automatically crop/resize the uploaded images to fit the sizes you have registered with add_image_size

NOTE

Images that already have been uploaded won’t be affected by adding add_image_size only new uploads will. To overcome this without having to manually upload all images again, use a plugin like Force Regenerate Thumbnails that will recreate the images with all registered sizes done with add_image_size

Here’s how to use it in it’s most basic form, keeping the original ratio (no cropping, no distortion)

add_image_size( 'custom-size', 220, 180 ); 

Check out the link to the codex to learn more on how you can use the available parameters.