Reading your comment, I think I have an idea of what to do. I will explain that idea to you, but would not be able to show any code, because it is just an idea, never implemented it yet.
I would suggest that you setup your sites to lead images from Amazon AWS S3. And on each of your sites, you would install The WP Offload S3 Plugin, and link the media files from those sites to the images in S3, but to do this last bit (linking the images), you have to write a plugin yourself, that ensure the images ID’s and references are accurate to all three sites.
I hope this makes sense. If this was me, this is how I would tackle this issue.
I think you can still do it without the S3, but you would still need to write some hooks that run whenever a media file is uploaded, so you update links to those images correctly.
That said, I think using S3 is more efficient in terms of pricing.
Anyway, this is just an idea, you have to do a bit of planning and investigation first. I think it would take a month or 6 weeks to finalize by one preson.
Good luck.