Time machine backup

Hi, I have an iMac with a 500gb hdd and I have a 250gb external hdd which I backup (using time machine) onto. I already have an exclusion list and only backup what is really important but I keep making home videos and taking lots of pictures and now the 250gb external drive isn't big enough.

I have a 1tb hard drive and am going to purchase an enclosure for it, pending the answer to this question.

If I start using my internal hdd and 250gb external hdd to save photos, videos fils etc on. Can I use the 1tb hard drive using time machine to back up the other 2 drives?

That may sound like a dumb question, but I really know know the answer!