Opal - these formulas will give you what you're after, confirm with
ctrl+shift+enter

Low Average:
=AVERAGE(IF(N9:N154<=79,N9:N154,""))

Middle Average:
=AVERAGE(IF(AND(N9:N154=80,N9:N154<=120),N9:N154, ""))

High Average:
=AVERAGE(IF(N9:N154=121,N9:N154,""))

Hope that helps,