But I think it depends on what you define as running 3 instances of scrapebox. Like Bhopkins says he runs 4 instances of scrapebox on the $35 Xsserver. I couldn't even run 1 on that server. But to me running 1 instances is posting at 200 connections to a list of 500K-1million urls or link checking at 300 connections a list of several hundred thousand urls.
I would typically say 1GB of RAM per instance of scrapebox if your doing medium to heavy duty. With Scrapebox Servers you can run other stuff besides scrapebox, they are just pre-configured for optimal scrapebox use.
So it really depends on what you are doing with a instances of scrapebox and what you want to do. If your running on a 200Mb inet connection and a powerful pc, an entry level VPS is going to be frustrating.