Paula, I was the one to initiate the threesome ......our friend of 20 years had become much closer to me while going thru tough times in her own marriage. I came to find out that she had deep feelings for me for many years but refused to act on them because of her friendship with my wife. Since our marriage had grown stale, to the point of us taking eachother for granted, bickering, etc, I decided to propose we change something without getting divorced ourselves. Hence the poly. It seemd like a natural extension for me given the fact that I have always had many "girl" friends since my teen years. I have only been married the one time and never strayed. In researching the poly lifestyle it seemd that the communal living, the shared responsibilities, the deep friendship we all three shared was, the perfect storm so to speak.
My wife needless to say was shocked at the suggestion but I guess grasping at the opportunity to save our marriage also, agreed to give it a try. We do still deeply love eachother and want to grow old together, as it turns out with our best friend. Yes the jealousy has been a problem. She is now at 6 months coming to grips with it and realizing how happy we all three are has gone a long way to ease her fears, worries, etc. Feeling secure in this poly life has a lot to do with letting go of the jealousy, I believe. It's still a learning process for us all, and they both know I love them dearly. As other here have said, communication is the key. Talk everything out.
As for meeting others, I wouldn't know what to tell you. Been 30 years since I was out looking. Sorry I can't be more help there, but I have heard from others that you must be careful with who you meet online for all the usual reasons. Hope this helps.