Three random number, return nil?

I want three different number in 1 and 4, but this not work why?!

local function randomMode()
	local m = {}
	repeat
		local Mode = math.random(1, #Modules)
		if m[1] == nil then
			table.insert(m, Mode)
		else	
			for index, value in pairs(m) do
				if value ~= Mode then
					table.insert(m, Mode)
				end
			end
		end
		wait()
	until #m == 3
	return m[1], m[2], m[3]
end