You can make a box of special toys that are only to be played w/ during school time. That way they are novel and will (hopefully) keep the child busy while you are doing school. Also, you can try to include the 3 y/o whenever possible. I print worksheets off the internet for my 4 y/o to trace and color when I am doing school w/ my 6 y/o and it usually keeps him quite busy. You can expect interruptions. Lots of them :D If the 3 y/o naps, you can get lots accomplished then. You may need to spread your day out more, taking advantages of quiet time or do some "school" in the evening after the little one has left for the day.
Good Luck! I find the preschool age group to be harder to homeschool w/ than toddlers. They chatter so much and seek so much attention :)