It is adding the 3 year old to the mix that makes it the challenge. Even if it were a 5 and 3 year old, it would be more of a challenge.
Younger children need more help than older children. Older children can work quickly and they want to do so (especially children who go to school; in school, projects are made quickly). So how do you make learning time work when you have a younger child who needs help step-by-step and older children who want to fly through the activity?
There are a couple of ways you can do it. Here are some tips to make the whole activity more enjoyable for you (fewer "What now mom?" "Mom!" and so on). It will also help you to have less time where a child is just sitting and waiting to know what to do next.
We start learning time out each day with our learning poster and our calendar. I quickly learned that I needed to instruct the older children to not blurt things out when I say, "What shape is this?" I instructed them to wait to be called on. I ask a different child each day about different aspects of the learning poster. I try to make the questions suit their skills. So my three year old gets asked what the number is while my seven year old is asked to give me two parts that make up that number ("What are two parts make that up the number seven?" "5 and 2.").
I don't want my three year old to be sitting in the room while her siblings blurt out answers faster than she can think about the question. I want her to have the chance to think.
Have a Sample Made
This is something teachers do in school. They have a finished product made to show the children before they start. This really helps children have a vision for what you are trying to accomplish. If your activity is one where you want some creativity, be sure to stress that this sample is your project, and that the child is free to do XYZ to their own project. If you have gotten your idea from the Internet or a book, you can show your child a picture from that source instead of making your own ahead of time.
Give a List of Instructions
Walk through the entire process with your children--or at least as far as you think they can remember on their own. This will work for older children who will be moving at a faster pace. The 3 year old or younger will most likely need you to provide one step at a time.
Don't Stress Down Time
What we are trying to do is avoid long periods of down time, but it is not a bad thing to have some down time. Learning to wait patiently is a skill in and of itself, so don't stress if your child ends up needing to wait for a few minutes while you finish helping another sibling. You just don't want constant waiting and constant down time. That leads to boredom and possibly "creativity" being channeled in ways you don't prefer.
Have Enough Supplies
Have enough supplies on hand that a child isn't sitting and waiting for long periods while other children use a certain supply. I try to have a tool per child. So each child has his/her own glue stick, crayons, scissors, etc. There are things, however, that must be shared due to economics and my lack of desire to fill my house with 3 different collections of things like glitter.
Have More Than One Activity
This is something I really focus on for my children. I like to have a few activities ready to go. One will be some sort of art project that I know my three year old will take longer to complete than my seven year old. Others will be activities I can have my seven year old do after he is done with his project. This can be sorting activities, sensory boxes, math worksheets, reading, etc.