1. You'll be lucky to get through in only two. There's a one direction thing, and an eVIP only survives 3 segments anyhow.

2. You'll have to do immigration at DFW. Make sure to have a visa (unless you're a citizen of a visa waiver country, since you're leaving from the UK i'm gonna guess you might be :P). You will also need to ensure you've completed your esta application. (http://esta.cbp.dhs.gov/)

3. assuming you're flying all AA metal, or some OW metal, you should be set. Just make sure to get the desk agent to check your stuff thru to GRU.

Safe travels!