How can I take a part of a video and chroma key only the parts that I select?
This isn't chroma keying. If your subject isn't in front of a green screen (or blue) then you're only other alternative is to mask it, rotoscope to be precise. After Effects does have a tool called a rotobrush that let's you brush around the edges of your subject but it's not the most precise or clean mask.It will however make for a very quick rotoscope. If you go frame by frame, you can make a mask with the pen tool and key the shape each frame as you adjust it. 72 frames at 24 frames per second would give you three seconds like you're estimating. That's about 2 days of work for someone with a decent amount of experience with the pen tool. Hope this helps.