The throw from third to first is usually measured in MPH, unlike the "pop" from catcher to second.
The high school coach probably doesn't have a radar gun so they time the throws with a stop watch.
But, the coach could use the pop times from third to first for all the players to still find the strongest arm on the team.
You could also take the "pop" time and convert to MPH by doing the math.
I can't remember on a Friday night what the distance is from 3rd to 1st. It's the same from home to second. 122 or 132, just can't bring it up.
It's difficult to say what MPH would be good for a high school player. It could be very different for a high school freshman to a senior.
I think pro scouts would start to take really take notice at 85 and over.
Less than 85 might still be noticed depending on the size and maturity of the player.