It certainly sounds like you've got your hands full! In terms of your toddler, it sounds as if he's biting because he can't express himself through words yet. Have you got any friends/family members who can help? Although I've only got one child, I get a lot of help from my family and friends. Can someone can help you with your children? If one week your friend babysits the twins, then the next week your parent/sister takes care of your toddler for a few hours, you'll have more time to spend with each child individually. This one-on-one time would probably help restore the balance for you.
i'm having trouble getting a good balance of time spent with each of my children...it sometimes feels like i'm getting there and someone rebels or the twins change their routine.my toddler recently bit his sisters face...19 months...he usually is the best behaved and easy going child...my oldest daughter 32 months...just stops doing all the good things she was doing...and i feel like tearing my hair out!i'm single and slowly going insane...