OK. Gosh. So you're both quite young, being responsible, planning to get married as soon as you can afford it.
You're working hard at jobs on different shifts, to save up for the wedding of your dreams. Then Miss Rock and Roller steps into the picture, joins your bf's band, gets to see him at practice, and then they both also choose to spend online time together during the rare time you and your bf have together.
This is not polyamory, this is your bf getting tired of acting like an adult and wanting to have a bit of fun on the side.
If it were me, I'd request he stop all online communication with her unless it's the simplest thing like when the band is going to meet. No dates, no kissing, no talking to her when it's one of the rare times you and he have together. You 2 need to talk, and have dates, and make time for good sex, and all the other stuff you did back when you were first falling in love. If he can't or won't agree to this, he probably doesn't want to get married. He doesn't deserve your trust and commitment.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years