Scripts

If the slot is empty, then the corresponding element is not nil, but 0. Therefore, ~=nil should be replaced with ~=0.
The idea is to remove one-third of the army? Then there is a shorter script.

for i=0,6 do
  local t, n = GetObjectArmySlotCreature(heroname, i)
  if t ~= 0 then
    RemoveHeroCreatures(heroname, t, n/3)
  end
end