Hello Goutam,
the limit for ATM OC-3 is two, we can guess a C3845 should be able to support 3 DS3 network modules.
see
http://www.cisco.com/en/US/prod/collateral/routers/ps5855/product_data_sheet0900aecd80300024.html
A maximum of one ATM OC-3 network module is supported on the Cisco 3825; the maximum for the Cisco 3845 is two.
But probably a safe choice is two DS3 to be sure the router can support them well full rate and with added features.
Hope to help
Giuseppe
