How to stop create unused thumbnail images in wordpress?

I need featured images with 3 different sizes, which I have working with this code:

add_theme_support( 'post-thumbnails' );
add_image_size( 'thumb1', 700, 300, false );
add_image_size( 'thumb2', 300, 100, false );
add_image_size( 'thumb3', 60, 60, true );

But I need these 3 sizes to only apply to featured images, not to images placed in the content of the post.

The above code creates 3 unused thumbnails for each image in the post and is filling up my server!

How do I code this so that the thumbnails are only applied to featured images?