Hi Rajitha,
I see--thank you for the explanation. The Campaigns area on the left navigation shows only the CURRENT campaigns where the lead is listed on a STATIC Marketing List. Because it does not show Campaigns related to DYNAMIC Marketing Lists, and since leads can easily be removed from Marketing Lists, I would not recommend using this as a reliable source for tracking the campaigns.
The Source Campaign is a good field to fill out, but it can only point to ONE Campaign. Therefore, if you want to track MULTIPLE sources, I would recommend creating a Campaign Response for each new Lead. In this way if you merge two leads, the two Campaign Responses will still show in the Activities. The Source Campaign will only show the value from the primary record that was selected when you merge.
You can automatically create a Campaign Response when the Source Campaign is filled out using a workflow.
For more information on Campaign Responses, check out this article.
www.powerobjects.com/.../using-campaign-responses-in-crm-2011
