Models like the D7100 have the C3 menu for the timer to take multiple shots, but the D3100 does not. And some accept a remote shutter button (remote control), but I think the D3100 timer will only take one picture.
EDIT: This would work, for $16.
http://www.amazon.com/NEEWER®-Digital-Control-Cameras-Compatibility/dp/B0044QMIH4
It is a programmable timer that plugs into the D3100 MC-DC2 port (compatibility shown on D3100 manual page 177).
It can do many things, including taking a few pictures at a specified time interval (programmable).
It would be used instead of the camera timer (it does its own timing).
It is an exact copy of the Nikon MC-36 (except is compatible with the D3100). I have the same thing (a different rebranded import brand, for a different Nikon camera port), but it works very well, and is inexpensive.
Camera auto focus would be different.
The camera timer focuses when you first press the shutter button, and uses same focus when the timer trips.
This one would focus when the timer tripped each time (which is better, if it can find you).
Both ways, if you are not standing where it tries to focus, and it has nothing to focus on, it could fail to activate.
Manual camera focus would always work, if correct.