Don't try to convince them to date other girls if they don't naturally want to, in case you've been considering that as a way to help them understand or to make rhings "even". Pushing mono people to be poly doesn't seem to go well. Mono's dating poly's, now that is hard but certainly *can* go well. If you feel like they don't understand, maybe ask them if they would read some of the essays on www.xeromag.com?
There is some great, very helpful stuff on there.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.