Making things more clear
To answer some of the questions, my feelings have greatly deepened and when they did the jealousy started to grow as well.
The jealousy was there even before he met the third girl although it does make me a bit more worried now.
When it comes to something happening in life though, he did get a second job (which he is going to quit soon) and with me working and Sam as well I felt like I didn't see him as much but Sam saw him more, which I found out later was the opposite.
I'm sorry for being so vague so let me explain more. Tim and Sam are married, and Tim knew me when Sam and I hung out and school and she introduced me to him. Back then Tim only thought of just being monogamous until we started falling in love then Tim watched an anime about poly relationships and that's how it happened. I knew Sam and Tim for four years but tim and I became a couple last year.
Recently though in the past few months the jealousy thing grew, I think part of it is because I used to spend so much time with Tim before he lost his old job and we both had to get new jobs and work longer to pay for the new apartment. I also love him so much that lately I think I am scared that such an amazing relationship won't last, I don't know what else is bothering me but in truth I am so scared that I will land up being forgotten because it happened a lot to me in the past with friends. I should know now though how much time he spends with me, and how caring he is so I don't know why I am often paranoid or jealous when I see how much he loves me.