I know you wanted opinions on the three sexual practices that you mentioned, but I'd like to share what I think are biblical principles that, when applied, allows these things to kind of fall into place. Rather like when we practice what Jesus says are the most important commandments: Love God with all one's heart, mind and soul and to love one's neighbor as oneself, then the 10 commandments pretty much take care of themselves.
The first principle of Christian marriage is that when believers join in maritial intimacy, the Holy Spirit is also joined as our bodies are the temple of the Spirit. Therefore, whatever we practice as man and wife, we need to remember to honor the Holy Spirit as well. (1 Corinthians 6:17-20)
Secondly, the Bible condemns lust. Therefore any type of sexual practice that serves just to satisfy the lusts of the flesh should be avoided. If there is something that one partner feels uncomfortable or even degraded doing, but the other partner insists upon doing it, then one has to wonder if there is a lust problem. We are to love one another and be joined as one, not use each other to satisfy lustful desires. (Romans 13:13-14; 1 John 2:16)
Along with that thought, the Bible explicity tells us that the husband is to fulfill his duty towards the wife and the wife towards the husband. (1 Corinthians 7:3-4) Maritial intimacy should be guided by the partners seeking to lovingly satisfy the other. So, again, if a sexual practice is engaged in because one partner is seeking gratification for their own fleshly lusts, this violates the purpose of maritial intimacy.
I am a BIG believer in Christian liberty. I truly do feel that as long as we follow Jesus' greatest commandments mentioned above, then we're pretty free to do what we want. This applies to marriage as well, only in marriage, the second of the two great commandments should be applied towards one's partner. As a Christian wife, I must first love and honor God and love and honor my husband. And, as a Christian man, he must first love and honor God and love and honor me. As long as we are doing these things, then I think we can practice what we both agree to with a clear conscience.
Oh, and yes, a 'threesome' is most definitely an adulterous and ungodly act.