Three, especially for a boy, is probably too young for doing worksheets. We started with other sorts of lessons.
Such as sitting together while I read stories. (Stories can help motivate them to want to read, introduce pre-reading skills like recognizing rhyming words, and can teach concepts too--especially if you read from a wide variety of books including nonfiction ones.) I found that if I read to him about castles and knights, he'd then spend time pretending to be a knight and seemed to learn better by acting it out. Or I'd read to him about fire fighters and he play act at being one of those.
Doing crafts. Painting, playing with clay, coloring, drawing, cutting and pasting, stringing beads, etc. These are not only fun, but also build fine motor skills to help ready him for writing. And they can help him learn to follow directions.
Doing science. Plant seeds and take of them to learn how they grow--if you're lucky, you might get them to reach harvest and he can learn where some of his food comes from. Collect seeds or leaves and compare them with each other to learn about botany. Try mixing jars of colored water to learn how colors combine. Get inexpensive science kits and build models of things like the heart or the skeleton or a mummy or whatever--they can be great fun, and help build knowledge that will help in learning more advanced material later on.
Disguised learning. Watch some fun videos. "Leap Frog" and "Between the Lions" are great series for building phonics skills. "The Magic School Bus" and "Bill Nye, the Science Guy" are fun science series. There are lots of others out there. And lots of libraries lend out copies of them. Watching some fun things like that will help him to learn, without it seeming like learning.
Exploring the world around him. Go on nature walks. Take time to watch the ants at work or to figure out where the bird you see flying about has his nest. Search for rainbows after the rain. And talk about what you see. Take him on errands with you and talk about the people you meet, places you go, and what they are for.
Anything you can do at this point is educational. But keep it fun. You don't want him dreading the idea of lessons. If you are going to homeschool him, you'll most likely be able to get all of the subjects done in an hour or 2 when he's school age. It's faster because there's less wasted time. So, don't worry about training him to sit still for hours at a time.
If you want some ideas on what to do, try http://www.worldbook.com under parent resources look for "typical course of study". You can start with their preschool list of skills that kids at that age are learning. But remember, at this age, *anything* you teach him is new to him. You don't need a formal curriculum. Just keep exposing him to new ideas, keep him mentally stimulated, and read to him a lot. Reading to him is the best thing that you can do to jumpstart his education.