How do I make a tycoon drop part script?

You can make a part with a script in it that creates parts for you. This should work:

local interval = 3
local function drop()
    local part = Instance.new("Part")
    part.Size = Vector3.new(2, 2, 2)
    part.Anchored = false
    part.Position = script.Parent.Position
    part.Name = "MoneyDrop"
    part.BrickColor = BrickColor.black()
    game:GetService("Debris"):AddItem(part, 120)
end

while wait(interval) do
    drop()
end