do's and dont's for a plural relationship

My wife and I both want to be as prepared as possible for bringing in another woman into our lives. We have prayed to God first and if He allows this and wants this for us, it WILL happen. So in trusting in God’s word of “Ask and you shall receive”, we have put our trust in Him and know that He will answer our prayers.
In preparation for this to happen, we have our home completely ready for her. All our finances are in place for two of us, but not for her yet. We are not sure how to go about some kind of agreement so that the three of us are completely and totally equals and yet still protecting the two of us from any financial or emotional disaster.
Can someone help us with the do’s and don’ts of a plural relationship from your point of view? Are there any books written on this subject?
Thanks to anyone that is able to help us….this is our first time and we DON’T want to fail.