New DAG Setup Help

I have a total of 3 sites, 2 in the US and one in the UK. I'm wondering if I am headed in the right direction with my DAG setup.  3 exchange 2010 servers (SP2) with all roles installed (1 server per site).

File share witness is located in the datacenter at Site 2.  

Create a DAG for Site1 and Site2. Passive database for Site1 in Site2, nothing from site2 in site1.

Create a second DAG for site2 and site3 (active/passive).

Or would it be best to create a single DAG for all sites?  I'm not sure I can do this as I recall reading something today stating each DAG would need its own replication network.

Let me know what you guys think.  If I am completely off my rocker here, please point me in the right direction.  I've read quite a bit today and I think I am starting to confuse myself.

Site 1: UK
MAPI network: 10.5.124.54
DAG network: 10.5.128.2

Site 2: US East Coast
MAPI Network: 10.5.14.4
DAG Network: 10.5.15.2 (for uk)
DAG Network: 10.5.18.2 (for us west coast)

Site 3: US West Coast
MAPI Network: 10.5.74.4
DAG Network: 10.5.69.2