This may be a case where the older two will have to do as much independent work as you can get them to do while the toddler is up and at 'em.
I know how hard that can be! My ds was SO distracted by any little noise or music, or a bird flying by the window.. you name it, he had to look up and see, or walk over to see... sigh.
It took a long time for him to be able to sit and do his work regardless of what was going on in the house, and he didn't even have a toddler to blame.
So.. if you can get the older ones busy with their work??? and you tend to the toddler... ??
Then use the toddler's down time.. "quiet time" or after he's in bed at night.. or have your dh give him his bath and story book time??? THEN you can do more seat work with the others?
Just throwing out some idea's here.
If he'll sit in a high chair, use it for "fun" activities for him while the others are doing their school.