I'm still confused by this though. Their market cap right now is about 1.5 billion. If GTA 5 sold $1 billion in 3 days, does that mean their whole valuation is based on this one single game??

Valuation includes liabilities and is generally focused on profits not revenue (Amazon excepted). Take-two ended up losing 32 million last year on 1.2 billion in revenue.

Take-two had a relatively large run up in July/August likely meaning that optimistic interpretations of GTA V were already priced in meaning people already factored the higher than normal profits this year.