Full code

local round_status = 0
local activeRound = 1
local t = 0
local interval = 2
local zombieCount = 3
local isSpawning = false
local spawnPos = {
Vector(-100.734840,-1274.259888,-79.968750),
Vector(-1803.711670,-1491.910645,-79.968750),
Vector(-1890.346436,975.450745,-79.392471)
}

util.AddNetworkString("UpdateRoundStatus")

function beginRound()
	round_status = 1
	updateClientRoundStatus()

	isSpawning = true
end

function endRound()
	round_status = 0
	updateClientRoundStatus()
end

function getRoundStatus()
	return round_status
end

local mextWaveWaiting = false

function getBestSpawn()
	local bestSpawn = Vector(0,0,0)
	local closestDistance = 0
	if table.Count(ents.FindByClass("npc_zombie")) == 0 then
		return spawnPos[math.random(1,table.Count(spawnPos)]
	end

	for k, v in pairs(spawnPos) do
		local closestZombieDistance = 1000000
		for a, b in pairs(ents.FindByClass("npc_zomebie")) do
		if b:GetPos():Distance(v) < closestZombieDistance then
			closestZombieDistance = b:GetPos():Distance(v)
		end
		end
	end
	if closestZombieDistance > closestDistance then
		closestDistance = closestZombieDistance
		bestSpawn = v
	end
end

hook.Add("Think", "WaveThink", function()
	if round_status == 1 and isSpawning == true then
		nextWaveWaiting = false
		if t < CurTime() then
			t = CurTime() + interval

			player.GetAll()[1]:ChatPrint("A zombie has spawned!")
			local temp = ents.Create("npc_zombie")
			temp:SetPos(spawnPos)
			temp:Spawn()
			temp:SetHealth(50 * activeRound)

			zombieCount = zombieCount - 1

			if zombieCount <= 0 then
				isSpawning = false
			end
		end
	end
	if round_status == 1 and isSpawning == false and table.Count(ents.FindByClass("npc_zombie")) == 0 and nextWaveWaiting == false then
		activeRound = activeRound + 1
		nextWaveWaiting = true
		timer.Simple(10,function()
			zombieCount = 3 * activeRound
			isSpawning = true
		end)
	end
end)

function updateClientRoundStatus()
	net.Start("UpdateRoundStatus")
		net.WriteInt(round_status, 4)
	net.Broadcast()
end