Filming with a DSLR

The 3100 has liveview right?  If so, then shoot in manual.  This allows you to change you shutter speed and ISO and you can see the exposure on the screen.  You can also switch out of liveview, change your aperture, and then back to liveview to see the change.  Those three things are helpful in setting exposure right.  Also I'd recommend a polarizer on very sunny days.  Another thing is you might be getting sunlight in your lens and it's washing out the picture.  You might need a lens hood of some sort.  Check these things as they're fairly easy to quickly check.

to avoid washing out on a sunny day, lower ISO, stop down some f stops with aperature (basically from f3.5 to f11 or f16 in that area somewhere) and for video i usually keep shutter speed about double the frame rate. so for 24fps about 1/48 (or 1/50) and for 60fps about 1/120 or close to that. for video it gives the most natural motion blur thats comfortable to watch. anything lower or higher will look "weird" unless that is the style you are going for or plan on doing super smooth slow mo

it has to have liveview for video. usually a lens hood is for preventing glare or lens flare, not controlling exposure.

OP, when you set your exposure, make sure to expose so that your snow retains detail, even if it means your riders are a little dark. if you're not shooting on manual (which you should be), then hold down your "+/-" button and set it to like -1.0 or so.