What type of special things do you do for an upshern? That is if its your minhag or are invited to one.
My niece recently made a fun set of tzitzit that would be a great project to do with an almost three year old or as a gift for the three year old. It was a commissioned request that required trucks, name and aleph bet. Any design could really be done based on the interests of your three year old in question.
Learn more about the tsitsit and see more pictures here.
These can also be used for starting school or other happy occasions.
I know that some people have a minhag of taking a 3 year old boy to their local Rav for a bracha at this time. I think that if it were me, I would rather take the child to the local Rav to ask a question. In that way setting the stage for asking questions in the future. What do you think?