This is how it works with your test command
function round(num, numDecimalPlaces)
local mult = 10^(numDecimalPlaces or 0)
return math.floor(num * mult + 0.5) / mult
end
RegisterCommand("test", function(source, args, rawCommand)
local input = tonumber(args[1])
local rounded = round(input, 3)
print(rounded)
end, false)