What is wrong with my script to change game mode on >= 3 players?

if #game.Players:GetPlayers() > 3
    script.Parent.TextColor = BrickColor.new("Really red")
    script.Parent.Text = ("False")
else script.Parent.TextColor = BrickColor.new("Bright green")
    script.Parent.Text = ("True")
end

You set the plr variable to the value of game.Players.NumPlayer. It's never going to change because it's just a copy of the number. In your while loop, you should be using game.Players.NumPlayer instead of plr.

However, there's an entirely better way to do this. Instead of polling every 0.2 seconds, you can just update the text whenever a player is added or removed from the Players service.

local Players = game:GetService("Players")

function updateText()
    if Players.NumPlayers < 3 then
        script.Parent.TextColor = BrickColor.new("Really red")
            script.Parent.Text = ("False")
    else
        script.Parent.TextColor = BrickColor.new("Bright green")
        script.Parent.Text = ("True")
    end
end

updateText()
Players.ChildAdded:connect(updateText)
Players.ChildRemoved:connect(updateText)

Also, to get the number of something, you generally need to put a hash tag before the value, like this.

local Players = game.Players
if #Players < 3 then
-- CODE HERE
end