polyamory

Polyamory literally means “many loves”, and covers a whole range of relationships and relationship systems based on the concept that one can love (or ‘be in love with’) or relate intimately and ethically with more than one person at the one time.
The relationship configuration referred to by the French as ménage à trois is known by a fair few people, but polyamory is highly developed in its dealings with communication, emotions (particularly jealousy) and the dynamics of relationships.
Even if you aren’t polyamorous, it is worth studying the techniques and knowledge of polyamory for the sake of the personal growth and learning you will gain.

There are considerable resources on this topic available via an Internet search - for instance:

• https://en.wikipedia.org/w/index.php?title=Polyamory&oldid=727755689; 
• http://www.polyvic.org.au/glossary; 
• https://www.morethantwo.com/polyamory.html.