|To return the highest number, try either MAX(A1:E3) or LARGE(A1:E3,1)|
Before we can average the 2nd and 3rd highest numbers, we may need some more info.
What happens if the highest number repeats itself?
e.g. If the highest 4 numbers are 50, 50, 49 and 48 would you want to see 49.5 or 48.5 as your Average of "the two second highest numbers"?
If the highest number will never repeat itself anywhere in the range, or if you would accept 49.5 as the result of the example above, then this should work: