I think the baseline is to split it equally, 33%, unless there is a very good reason that one of the three collaborators deserves more or less than that. Greed is not what is motivating me at this stage in my life

How do you intend to use your own money to fund this startup, then? Unless you intend to just give a straight loan to the company to be paid back with interest at some future date, then this will eventually be an unequal split. That's fine. But just be careful about how you do this.

I'll buy preferred convertible equity probably in addition to common stock, so that I'm both an investor and a founder

That's a sensible approach. Of course, then you won't be entirely equal in terms of your ownership in the company once the security converts.

One more thing: So, you have a 33-33-33 split. That means you can be fired, unless you build in protections for yourself. You don't want to put in a bunch of money into a startup, get fired, and have nothing to show for it.

I'm not too worried about this at this stage. But I agree with you