Highlighting at home has come a far way in the last few years. If you're not afraid of the results, because it can easily go wrong, go for it. Personally, I have never had a chemical process done by a professional.
If you have dark dark hair, you'll most likely have to take two rounds of bleach if you're using any old drug store brand. Leaving it on for 3 hours isn't going to do anything more than leaving it on for 90 minutes. After a certain amount of time the bleach dries out, and is no longer active.