A 3-bet is a preflop re-raise over an open: the big blind counts as the first bet, the open as the second and your re-raise is the third. 3-bet % measures how often you do it when you have the chance.
How it's calculated
The healthy ranges on the side are guidelines for NL Hold'em cash games with 100 bb stacks, the same ones Flopmortem compares your game against. Use them to spot big deviations, not as an exact target: they depend on the table, your opponents and your style.
How to read your 3-bet
With a low 3-bet, opponents open against you without fear and you play too many pots by calling, without the initiative. The usual approach mixes value 3-bets (QQ+, AK) with some quality bluffs in position, like A5s.
A very high 3-bet costs money too: observant opponents fight back with a 4-bet and you lose the pot with your weak hands. Look at it alongside your fold to 3-bet, the other side of the coin.
If your 3-bet is out of range
Too low
You 3-bet too little: opponents open without fear. Add value 3-bets (QQ+, AK) and some bluffs in position (A5s, for example).
Too high
You 3-bet too much: when you face a 4-bet, you lose the pot with weak hands.
How many hands you need to trust it
Flopmortem doesn't flag your 3-bet as a leak until it has at least 80 opportunities. From there it calculates it with its 95% confidence interval: if the whole interval falls outside the healthy range, the leak is confirmed; if only the measured value is outside, it's a trend. The guide how many hands you need to trust your stats explains it.