What is your idea of acceptable amounts in laning phase?
It really depends on what you got for the deaths, as well as how long the lanning phase lasts. Also the Elo of your opponents, a higher ranked player will often push their advantages harder.
But from personal experience, 3 kills in 10 minutes with 0 deaths is pretty much the point of no return on any kill-based champion (fighter, assassin, mage, marksman).
Their lane opponent no longer has the capacity to stop them, and need a team-wide effort to prevent the juggernaut they've created.
However - going 3 kills and 3 deaths is no real ground being won or lost, making 3 deaths vs. 3 deaths extremely different based on the circumstance.
Ultimately, it's more about the proportional difference in power that's created from the gold/experience disparity that makes giving kills sting. And if you give up a Giant's Belt/B.F.Sword/Needlessly Large Rod when you're just rocking a Doran's Shield/Blade/Ring, you're not going to have a good time.
Anyways - I'd say being down ~2 deaths over your opponent's is acceptable.