Yes you'll certainly lose weight walking 3 miles per day.
When you walk at a brisk pace for just 1 hour you can burn up to 400 calories if you walk 1 mile in that 1 hour.
So in 3 hours you should walk 3 miles which would allow you to burn off 1,200 calories or 3,500 calories in 10 days times of walking 3 miles per day.
Walking just 1 hour per day or 1 mile per day is usually good enough to bun off enough calories and lose the weight you desire but if you can walk 3 miles per day then that's much better.