if game.Players.NumPlayers >= 3 then for i = 30,0,1 do script.Parent.Text = "Intermission"..i wait(1) end end
You're attempting to count up from 30 to 0 by increments of 1, which makes absolutely no sense. Change the 1 to -1 to make it count down instead.
if game.Players.NumPlayers >= 3 then for i = 30,0,-1 do script.Parent.Text = "Intermission"..i wait(1) end end