D.K.
The three year old needs to be working on things like counting, letters recognition and sounds. You can just 'PLAY" together and make it so that the little one AND 3 year old get something out of it.
i.e. you are playing w/ blocks....the little one is playing, building, etc. the blocks. If the blocks have letters, ask the 3 year old WHILE you're playing what those letters and sounds are. Ask the little one to hand you a "ball", "truck" or whatever. Ask the THREE YEAR OLD what letter/sound that begins with. You are playing together but making the same play benefit both children. The little one is working on object recognition, the older...letter and sound. you can make pretty much anything like this.
Grocery store.......litle one recognizes the banana, 3 year old tells you what letter and sound. EVERY situation can become a learning opportunity if you just get creative. I taught addition and subtraction by counting stoplights TO church and subtracting coming home, etc. (I'm a former pre-school teacher.)
Cooking.......let the them experiment - have opportunity to touch, smell and taste various foods to determine whether they are soft, bumpy, lumpy, sweet, spicy, etc. I used food to educate them on geography - WHERE they grew...Science - HOW they grew, etc.
use the calendar and let the older one go thru the days of the week (one each day) and learn to recognize the days of the week, etc. I had 3 & 4 year olds WWWAAAAY ahead of most preschoolers when it comes to reading, phonics, etc. You just have to get creative and use every opportunity to teach. Just a new way of thinking for most people, but NOT at all hard.
Putting on shoes........ask the little one to GET the shoe or point to, ask the older one what letter "shoe" begins with. KIDS LOVE TO LEARN, EXPLORE AND DISCOVER. The more you work with them at this age, the more ready they will be for school. Get them while they are anxious to learn and discover. AND........I'm ALWAYS saying, "It's so much fun to learn new things."
Hope this helps.