WordPress 3.0 Plugin Activation Error – “Headers already sent”

Well, i have been customizing my WordPress a lot to produce something like the food directory or blogshopping tool which required a lot of hack on to WordPress to make everything work perfectly. Recently i have upgraded my WordPress to the latest version 3.0.1. Everything seems fine until one day i decided to enhance my site. Upon activting my WordPress plugin, an error message occurs stating "The plugin generated 3 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.". Although the plugin successfully activated, it seems like there are some problem with the plugin that is causing this. I search high and low for it but couldn't seems to detect any header being sent explicitly without my knowledge.  To make matter worst, this caused all my timthumb (image of the fly) script to malfunction which caused ALL my images to break. Hence, none of the images generated by timthumb were generated on the website. This is disaster!!!  Why is my header being sent when there is NOTHING in my code that is sending it?!!! (panic)