I know how you feel, we are raising our 4th now, so we've gotten a bit better at this over the years. You can't just say that you are going to take away privileges, you have to back it up.
Tell them that if it isn't done then you will take their phone for a week,(if they have them). Or take something that means a lot to them. If they have plans for the weekend, cancel them, and stick to it.
Also, if you think a reward might be easier, take them somewhere they want to go, but tell them that they have to keep up to their end of the bargain.
Our last one likes to ride her bike to and from school on Friday.
She knows that if she is late for school, or tries to give the teachers a hard time, then she doesn't get to ride it. Hope this helps.

 I am in school and my parents cant really help me either. All your kids have to do is concentrate in school and take notes. My parents told me a lot of things to do to make sure i understand what to do. Just make sure that they pay attention and help them as much as you can. At the next parent teacher conference (if you have any) ask the teacher about it. Ask him/her how your kids are doing. If you don't have conferences then try and call the teacher. If you cant do anything like that then just make sure that they pay attention and take notes on how to do it.

 Is the problem that
a) You don't have time or
b) are having difficulty comprehending it yourself.
I have a 2nd grader and sometimes her homework is a bit tricky. But I think in any case there should be some sort of tutoring program at school. Check it out and get your kids in there.

 With the passage of time, children need different types of help in different timings. Parents are so much busy in their practical life to cope with the financial responsibilities, which are imposed on them, that they cannot spare enough time for their children. So far your problem that you have three school-going children who need your help and you don’t have time for them and you want a reasonable solution for that.
First of all try your maximum to spare your time in the evening or at night and help your children in their studies and homework. Suppose if you don’t have a bit of spare time then consult some of your friend or relative to find a good devoted teacher for your children who could guide them in their homework as well as in their regular studies. I know it is materialistic age and people work only for money but even then there is some devoted people who teach their students wholeheartedly. They not only consider teaching their profession but also a noble job to do. I hope if you are able to find out such a teacher, your problem will be solved.

 Jasminetq is right by telling you about the tutoring programs. and also Your kids education come first before house chores. There is always time for that But your kids come first!!! All school Districts have Para professionals that work with students one on one speak with there teachers and set it up there is no cost to you. You Pay for it with your taxes.

 if you are having trouble comprehending with yourself then most likely you should talk with your child's or children's teacher and ask them how they expect your children to do this assignment when you as an adult can not complete it and remember not to get agitated with either your children or the teacher when discussing this occasion.

 Well i suggest a tutor it helped with my children and now they are the most brightest students in the school honest.

 Since you have a computer there are a lot of sites to help you help your kids,plus you could call some of their classmate's parents and see if they can give you some input.i am raising a 11 year old grandchild and this helps me a great deal.

 Have the other spouse or the oldest child help the smaller child that knows because that other child already gone threw that already.

 A Tudor, college kid, is always a second good choice.

 Well if you have three kids with three different ages one can help each other. You can help with your younger child while the older one can help the youngest one. Another thing is to enroll them in after school tutoring.