What's the meaning of Duration: concentrate+3 round?

[With a box]

is this mean It has 3 round of duration and it reset to zero if I use action to concentrate?

[eggynack]

It means that you get three rounds of duration no matter what, but that you can extent the duration by concentrating.

[ninjamaster1991]

It lasts for as many rounds as you concentrate (usually taking a standard action each turn, unless you take feats for it), plus three rounds after that.

[With a box]

It means that you get three rounds of duration no matter what, but that you can extent the duration by concentrating.

I failed to understand this in one meaning. can you make an example?

[Lord Lemming]

I think it means that it lasts as long as you concentrate, but it continues 3 rounds after you stop concentrating. So if you start the spell on round 1, concentrate on round 2, but do not concentrate on round 3, then the spell will continue functioning for rounds 3, 4, and 5, and then expire.

[Fax Celestis]

I concentrate on detect thoughts for five rounds. It stays active for that time. Once I stop concentrating on it, though, its duration changes to "three rounds" and acts like any other spell duration at that point.

[eggynack]

I failed to understand this in one meaning. can you make an example?
You cast it, and then it's there, as a great thunder. But, also as thunder, it is strictly temporary, set to shuffle along our mortal coil in naught but three rounds. Is this fate inevitable? Perhaps in the long term, but with sheer force of will, perhaps you can turn the tides. You harness your brain power to the vast spell network, and for the first time, it all seems so simple. You just concentrate, for one round, ten rounds, more? Perhaps. You lay your focus on it for so long, that the spell almost feels like a part of this world, and a part of you. But, it is not to be. You cease concentration for but a moment, and second by horrible second you can feel the force slip away. One round, and it begins to fade. Two rounds, and you shed a single tear. Three rounds, it says goodbye to you. Its first words. By the beginning of that fourth round, it is gone, never to be remembered except by those who were graced with its presence, and even they are temporary. You understand how much time means more than you ever have, but you still miss the spell. Perhaps you will cast it again.

[With a box]

So that's not good as I thought.