Assuming you can convince him to join at whatever stock distribution is acceptable to him, you, and your other founder, you might not need to worry about what's left for future investors. That will be a separate negotiation, and everyone will likely get diluted then, whether they own 5% or 50%.

3 founders can work fine. Many would consider it better than 2, all else being equal, since with 3 you can break some decision logjams. But that assumes all 3 are good, and that you have developed a strong working relationship. Reading your post, where you are technical, you are bringing someone on to help, and eventually he may be promoted to CTO, makes me wonder if you view him as more junior, currently. And whether he views himself more junior. That might drive both the equity split, and whether he participates as an equal in routine operational (or even technical) decisions.