What did Mikasa say when she was correcting Armin after Eren mastered the ODM gear?
This is the conversation in episode 3:

アルミン: 目で「どうだ！」って言っているよ。
ミカサ　: 違う。
アルミン: えっ？
ミカサ　: これで私と離れずにすんだと思って安心してる。

Literally, it translates to

Armin: (His; Eren's) eyes are telling &quot;How's that!&quot;.
Mikasa: (You're) wrong.
Armin: Eh?
Mikasa: With this, (He; Eren's) relieved thinking that (he) doesn't have to be separated from me.
The word in (parentheses) is the omitted subject.

Mikasa's words is a response to the conversation in dining scene (which was slightly omitted in the anime)
From post no. 382 on 2chan,

When Mikasa told Eren, &quot;You shouldn't aim to be a soldier anymore&quot;,
Eren, who couldn't dispute it, was staring at Mikasa annoyed while thinking &quot;I understand even if you didn't say it!&quot;.
Mikasa misunderstood Eren's expression as &quot;Looking from his staring, he hates to be separated from me&quot;.