1. It is not allowed for a Muslim man to have more than one wife if he is unable to implement practical justice between them. Allah says in Quran : ( But if you fear that you shall not be able to deal justly with them, then only one wife) ( Sura 4, Verse 3).
2. The Muslim man must keep the respect of every one of his wives and never hurt feelings of any of them.
3. The acts of threesomes is not allowed in Islam at all. Any of his wives is not allowed to see other wife during intimate situation.