I have a Nikon D3100. It has a built in flash. Can I use it as a light when filming videos?
Generally speaking, it is not possible to use a camera flash as a light for video. A true flash is a high intensity discharge bulb which produces a very short (1/100 second or shorter), very bright burst of light. It is not designed to be used for constant light output and has insufficient power, durability or cooling to be able to operate as a continuous light source.
Some devices, most often smartphones, use LED light sources as a "flash" but actually are not flashes, simply constant output lamps that can be used for video or as a low brightness "flash" for still images. This is very rare to see with a point and shoot or DSLR. When it is included with DSLR flashes, it is typically as a separate, distinct light on the same unit, such as the video light built in the Canon's 320EX flash.
As mentioned above, no. The bulb and circuitry is not set up for it.
If you are looking for a cheap solution to home video lighting, go to a hardware store and buy a set of Double Halogen Work Lights With stand. It will run you like $40 and is just as good as any fancy video light for basic stuff.