My favorite baseblogger at Bleed Cubbie Blue uses it frequently, but I can’t find it defined anywhere online. I think it might mean “to win a series”, but I’m not sure. Anyone heard this?
Meat Loaf song from 1977 - “Two Out of Three Ain’t Bad”
I’m a lifelong baseball fan, but I may not be hip to what the kids are saying these days. 
I’ve not heard it used in reference to baseball before (other than the Phil Rizzuto play-by-play in Meat Loaf’s “Paradise by the Dashboard Light”), but @Folly 's definition makes sense to me, and it fits with the idea of winning a series by winning two of the three games.
It means if you don’t commit to the game and promise to love it forever, you’ll never get to home base.
Let me sleep on it.
I need to know right now!
Shouldn’t you guys be over in Café Society? 
If you’re a White Sox fan does it mean you’re praying for the end of time?
That sounds right, but has anyone actually ever heard it used in this context?
No. It’s probably something specific to that blog.
As I noted, I haven’t heard it before, either. Could be that that particular blogger coined the usage, or at least heard it somewhere, and likes it a lot.
I found this, where Bleed Cubbie Blue says it’s a Joe Maddon phrase.