You're setting yourself up for a doosey of a failure, should it happen(it can). Funneling everything to "one" anything is bound to cause some issue to arise. I don't know why you have 3-servers but you've crossed into commercial or beyond typical home user application. Thus, you need to spend the $ to get real and find what it really takes. Why, you're not using some Server OS(or similar) is beyond me(I assume). I would think, your best doable and reasonable setup would be a dump to a singular HD for each server. But, IMO look at NAS setups and Buffalo can help there too. If you have a home brew server setup, try to standardize what you have, so things can settle. When it comes to USB, try to stay away from that, use ethernet when you can. Why, because think of it as a highway of 3-lanes now where everyone has to exit onto a 1-lane exit ramp. Plus, you made your setup to connect all to one backup, you've in effect made a backdoor link of sorts, somehow I can't see this as a good idea.
Have 3 servers all on separate dedicated private networks. These servers are not allowed to route to each other. All servers reside in the same rack. Would like to use windows server backup to create recovery images of each server and save to external. Was thinking about putting in a USB 3 hub then attach to a Buffalo TeraStation 5400r Rackmount via USB. Doable? (alternative suggestions would be greatly appreciated) Thanks.