Sometimes 2 doran's have to be enaugh, when your opponent plays really smart and you cannot kill him for long time or even you lose early. But in this case, infinity is way to expensive for you, it is better to tank earlier and farm hard, and maybe even ks in some situations (like your carry/lane partner is fed enaugh and one kill makes him/her no difference) to make your situation better.

I played with infinity hundreds of games. AD boost is huge, regardless you buy 1, 2 or 3 doran's (or even 0), but you are not ad carry! You should even try to avoid killing enemies, leave it for carry. Getting other kills than those you cannot leave for other champion only weakens your team in fact.

Going dps is ok only when you are going to easy win. Even not fed carry deals much more damage than fed Garen mid/late game.

garens never useless late game but great build otherwise :)

cool build, but I guess two dorans are enough, u doesn't nedd the third one. An infinity edge could fit well too bringing u more offensive power in some situations, EG: ur team already has another tank guy.