May I know if it is possible to migrate (or downgrade) from SharePoint 2010 to SharePoint 2007 or WSS3 and what are the steps in going thru this? (This is because some of our web part only support SharePoint 2007/WSS3 only and not both version and end-user need that functionality that the web part provided.)
There is no way to revert your content databases back to the 2007 level from the 2010 level. You would need to build your SharePoint 2007 sites from scratch and then migrate your content over manually. You can use commercial tools from companies like Quest, Metalogix, and AvePoint. For documents only, you can use some free tools like BitKinex to move them.
I don't think this is truly possible. What you will you need to do it is create your SP 2007 site and then manually copy all documents and list data to the new site.
Depending on the size of your farm, it would probably be easier to get the webpart working in 2010.