Source: http://win.wizkids.com/bb/viewtopic....cf3d0aba7deca8

SMOKE CLOUD POWER: Minimum range value 4. Place up to 6 hindering markers within range, each adjacent to at least one other, and at least one marker must be within line of fire. Opposing characters occupying a marker modify attack -1. At the beginning of your next turn, (even if this is lost) remove them.

If I have a character occupying a square with 3 smoke markers placed by 3 other different characters, would that character get a -3 modification to it's attack value?

WK: No. With terrain markers, only the topmost terrain marker affects the game. All other terrain markers underneath do not affect the game in any way, but are still (and can be) removed by effects.

In the provided example, the character is considered to be occupying just one Smoke hindering terrain marker, so it will only modify it's attack value -1.