A few notes:
If we know that little brother has a play date on Monday, then we may do Nature Study on that day because it requires much more time and concentration (harder when a 3yo is trying to take your paintbrush!). Much of the reading that requires careful listening and narration is in the afternoon, during little brother's nap.
Little brother does participate in Bible and folk song and will often sit for a few minutes at the table and watch brother and sister work. Mostly, he plays with his trains and other toys and waits for one of us to play with him during our "scheduled" play time. He participates in preparing snack and lunch, and he has his own morning jobs to do. As he gets older, he is more understanding of what it means to "do school." He asked to narrate a story a few days ago, and he has started to count (on his own) on the abacus during his play activity. Much of his learning is by osmosis. He sees learning being modeled for him by big sister and brother, and so he is coming to the table more often to see what is going on. He does not need to begin formal schooling, but I think it is valuable for him to observe that his siblings enjoy learning!