Most of the information you'll find on harmonious poly relationships online are going to be for Polyamory.
Polyamory is a fairly new concept coming about in the 1970's and only becoming known in the "mainstream" in the last 5 years.
Polyamory means one person (of either gender) having more than one relationship- whether sexual, emotional or spiritual. Polygamy is one man having more than one wife. Polyandry is a woman having more than one husband.
Multiple relationships require trust, honesty and open communication from all partners. There can be no resentment between partners lest strife come about and then relationships break down.
Really the fundamentals of multiple relationships are the same for monogomous ones. Balancing time between many different partners gets a little trickier but if you are open with all people involved you will usually be able to figure things out.