To raise track level 3 inches, regardless of scale, with a 3% grade, you would need 100 inches of track.
As sumpter250 mentioned, you could not make a helix with Woodland Scenics risers, since a helix runs in a spiral (like a spring) and the WS risers are solid, making their use in a helix impractical. If you're just going up or down,
Since you couldn't make a helix out of WS risers, a simple turn-back curve would require 100 inches of run to go up three inches--so, really, whatever radius you wanted, plus whatever lengths of track for the "legs" of your riser to total 100 inches.