try with:
{
_x spawn
{
{
_x action ["EJECT", vehicle _x];
unassignVehicle _x;
[_x, getPos target, 20, ""]execVM"mando_chute.sqf";
Sleep 1;
} forEach units group _this;
};
} forEach [chuteman1, chuteman2, chuteman3];
EDIT:
Missed the last parameter (chute type), corrected in this code.