You have a single-line, analog-connected (RJ-11) cordless phone. It is incapable of handling more than one telephone service at a time. You were unclear as to the details of your use of Google Voice, but one Gmail account can have one, or optionally two, inbound Google Voice phone numbers. You can't possibly have three different calls at once, let alone working on that telephone.
If you truly need multiple, separate telephone calls on three different handsets, you need to spend a lot more money. Since the majority of posters here are not interested in spending any money, I won't waste time typing out how that's done, unless you have the money (a couple hundred dollars, at least) to do it.