Of those three, Ada and Pascal are as I understand it pretty closely related syntax-wise (in the same "family"), so learning one of those will help a lot with the other.

I'd recommend starting with Pascal, because of the comprehensive and high quality standard libraries (or "frameworks" -- see related recent HN discussion...) in Free Pascal and Delphi, and because Free Pascal can target a lot of different OSes. OTOH, a downside might be that you learn (to rely on) too much of the FCL/VCL in stead of just syntax, so you get confused at the lack of those libraries in Ada; that could speak for taking it the other way around.

TL;DR: IMO, in a way that's perhaps closer to two new languages to learn than three.