Warnings: character death, mention of suicide.
A lot can happen in three years.
You can, with the right amount of talent and determination and mentorship, master senjutsu or advanced medical ninjutsu.
You can travel, make new friends, rent your own apartment, develop techniques, write a pornographic novel, grow a fifth of a metre, experiment with your wardrobe, acquire superhuman strength, and dine with toads.
It takes them three years and nine months to find him, chained, blinded, mind as empty as his eye sockets. It makes so much sense now — why no news of Uchiha Sasuke had filtered back to Konoha during all this time — because he never left. As his last eye finally shuts and they prepare to kill Danzo again (and again and again and again and again and again) he steps in the way of a kunai with eerie precision. For a second he is back, spine straight and knees locked, ("Sasuke!" "Sasuke-kun!") and then he is gone.
A lot can happen in three hours.
You can, with the right amount of rage and chakra, raze half a village to the ground or massacre an entire organisation.
You can desecrate corpses, boil lakes, torture the dying, melt monuments, plateau mountains, and evaporate forests.
It takes them three hours and fifty-two minutes to neutralise him, charred, raw, fighting tooth and nail even as parts of his body fall to the ground like crumbs. She has read about this, during those three years and nine months, about how jinchuuriki lose their sanity (humanity) if they go too far, about how seven tails is the limit. (She was always good at maths, and no matter how many times she recounted there had been eight.) Her hands do not shake when she drives a chakra-reinforced scalpel into then across his throat. For a second he is back, lips pliant and eyes round, and then he is gone.
Three seconds later, she leaves, too.