CSS3 Box-Shadow Linear Gradient?

Late to the party, but maybe someone will find it useful!
You can actually do it with multiple shadows on the box-shadow:

box-shadow: inset 0px 33px 25px 0 #000, 
            inset 0 66px 15px 0px #ccc,
            inset 0 99px 5px 0px #fff;

codepen example : https://codepen.io/InFecT3D/pen/JQdmeL

Side note: it might be a little “hacky” approach, but in some cases it helps.