Hi nappyhair,
Yep, I'm bilingual too, since before dd -- I became deaf as a teenager. English is my first language, ASL my second.
I think (hearing children of hearing parents) are most motivated to sign when that is THE way for them to communicate. When they can communicate as easily or even more easily by talking, there is less motivation to sign.
I do lipread, and my husband is hearing, so my daughter speaks a great deal. (Today at dinner she said, "I want a pet, but not an ordinary pet... I want a cat with beautiful long white fur." -- word for word transcription.
) I am in a tricky stage now where I don't want to pressure her to sign, but do prefer that she signs more than she currently does. The main thing that I've found helps is to sign MYSELF as often as possible, especially in fun ways like storytelling or having "secret" conversations at a restaurant.
That's so cool that your kids might be tri-lingual!
Very brief rant which I already mostly covered in the above post -- it took so long for ASL to be recognized as a real language, and in that context it makes me wince a little bit when it as seen as interchangeable with homesigns (made-up signs.) One is a language, a real language, one is not. That doesn't mean that there is anything wrong with home signs, really, just that they are another kettle of fish.