You can have more than one MA in a single indicator. Just copy and paste this into a MA script:
study(title="MA 50/99/200", overlay=true)
short = sma(close, 50)
longer = sma(close, 99)
longest = sma(close, 200)
plot(short, color = orange)
plot(longer, color = aqua)
plot(longest, color = blue)