I've been tabletop RPing for about 15 years. I'd like to get my three year old son interested in it.
Don't.
Seriously, as the father of a son who is angling towards the geeky...
don't.
Don't force it, don't push it, don't rush it.
Kids at that age don't need rules and structure to their games. That just imposes shackles on their imagination, which you don't want to encourage. You want their play to be free.
If you're interested in getting them into gaming and D&D, keep them adjacent to gaming. Let it be something in your life that they can see you doing and talking about and not some strange, hidden activity. Let it permeate their life.
Get them a copy of
The ABCs of RPGs and the associated
activity book. Get them a copy of
Dungeonology. Maybe a stuffed D&D monster or two.
Show them your books and miniatures. Look at the
Monster Manual with them.
Then make up stories with them. Just tell stories and ask them what happens next. Give them cues. Let them make up a character and tell you what happens as you collaboratively invent a story.
Later, you can move that to a tabletop and add minis. Perhaps treating them more like action figures and telling a story with those. The D&D board games are also good for this, as you can make up the dungeon and have lots of durable pieces to play with.
When they get to kindergarten or grade 1, then you can add more rules or bring in a more advanced game. But that works best if they have 2-3 friends they can play with.
When they are ready, there are games you can give to them games like No Thank You, Evil:
http://www.nothankyouevil.com/
(You can keep an eye on that for later or back the ongoing reprint Kickstarter:
https://www.kickstarter.com/project...l-a-game-so-nice-were-making-it-tw?ref=74r5a4)
What you don't want is to force it on them. Then it becomes that activity that dad/mom
made you do. They need to initiate the play. (Really... if you tell them they're too young and make it almost a forbidden activity at first, that just makes it more appealing later.)
You don't want it to become that thing they did when they were young and associate as a little kid game.
