I think it is hard but you can
do it! Your 9 year old should be able to do some work on his own. Your 6 year old should be able to complete his assignments within an hour or so. The rest of the day he could do fun educational activities with his younger brother (while you have one-on-one time with your oldest). Some ideas would be educational videos, learning games on the computer, crafts, educational board games, building toys like Legos, play dough, drawing, etc. Maybe you could type up a list of options for them and put it on the refrigerator. Then they could go to the list to find things to fill their day.
You may not cover everything but that's okay! There are many gaps even in traditional school. Your kids will learrn more than you realize! Reading books is the best way for them to be well-educated. The classics are especially good.
Here is an article that might be helpful to you.
http://smarttutor.com/home/Homeschoolin ... Levels.asp