Hello Mahesh,
1)
As you wrote there are some limits to the traffic control you can achieve
see section point-to-point and point-to-cloud
http://www.cisco.com/en/US/tech/tk828/technologies_q_and_a_item09186a00800a43f5.shtml#wp27879
with what site have primary site (site1 and site2) to communicate ?
Can we thin of primary site as a site with two CE routers that connect to the same customer site or are they distinct sites ?
2)
this can be made with different routing features like BGP conditional advertising for example.
or simply the backup site advertise the same routes as site1 and site2 but until one of them is up site3 is not used.
this can be achieved with some BGP tuning for example on local preference so that all other sites will use the sites.
You can build a hierarchy of routing both directions
site1 preferred both ends limited to 150 Mbps
site 2 second preferred
site3 used only if site1 and site2 are out of service /isolated (disaster recovery)
if you do so you gain control on traffic because only one site is used at a given time.
Again if site1,site2 are two CE nodes of the same site you can implement a hierarchy.
The customer can say that we would want to see both site1 and site2 used at the same time.
Some choices and tradeoffs need to be done.
Hope to help
Giuseppe