Are all nikon lenses compatible?
I'm about to purchase a Nikon D3100, I have an Nikon D70 and D300 with the kit lens and the telephoto lenses and I switch the lenses between the two cameras and was wondering if the kit lenses (18-55 mm) on my D300 would be the exact same lenses on the Nikon D3100? And if I can just buy the body and put my old lenses on it? I will mostly be using the new camera for video, would that effect the auto focussing at all?
Yes they are compatible. However, realize that the D3100, being an entry level camera does not have an in-camera focus motor. Therefore, if you have any AF (non AF-S) lenses, they will not autofocus on the D3100. They will work in all other aspects.
If you have AF lenses, then the D7000 would be the logical upgrade as it does have an internal focus motor (like your D70 and D300), and can autofocus AF lenses.
You already have a pro level APS-C D300 camera and a nice D70 as a backup. Why the interest in the D3100? It is as mentioned an "entry level" camera. I would think that selling your D70 and buying the D7000 would give you a better balance
This would probably answer your question more properly.
Since you can't answer this yourself, you don't need a third body (or even a second) and you shouldn't be the owner of a d300.
For recording videos, buy a camcorder.