I think there is a snow/beach scene mode on the 3100 if so use that.
If not set on program os S or A and increase the EV by + 1.5.
The meter sees ann average scene which when all mixed up equates to a mid grey nmand exposes for that. On a beach or in snow it is brighter than average so it will come out dark unless you inmcrease the exposure to compensate.
If you flick the joystick up you will see a histogram of the exposure. The graph should be in the middle. if to the left it is under exoposed so increase thew EV as above.
Finally, if you shoot RAW rather than jpg. it does not matter as you can then process the images after. Raw keeps all the detail whereas jpgs are processed and discard useful info that can be used.
Finally finally go to white balance take a snow shot and use that to make a pre set colour balance value for that shooting situation.
If that lot makes sort of scence, look it up in the manual or on You|Tube to qualify it.