How does Othello's language change in Act 3?

Act 3 of Othello is pivotal for the development of the play as a tragedy. This is the place in the drama when Othello finally starts to internalize the lies and deceit that Iago has been spreading against Desdemona. Iago is a master of language, using innuendo and insinuation to turn Othello against his wife. Indeed, he doesn't need to say much to create and heighten Othello's suspicions. In scene 3, for example, the most important in the whole act, Iago's casual response to Cassio's sudden departure is entirely characteristic:

"Ha! I like not that."

Later on in the scene, Iago stirs the pot even more, but again without explicitly accusing Cassio of cuckolding Othello with Desdemona:

"I cannot think it that he would steal away so guilty-like."

This is a classic use of irony by Iago. He appears, as is his wont, all sweetly reasonable and nonjudgemental, but we all know what he's really up to.
As Othello increasingly starts to fall under Iago's malevolent spell, his language changes as he is becoming ever more intemperate and expressing his growing insecurity and lack of trust. Othello's confidence has been so undermined by the drip feed of Iago's subtle slander that he's almost starting to resent himself:

“O, that the slave had forty thousand lives! One is too poor, too weak for my revenge.”

Othello doesn't simply want to avenge his honor by destroying Cassio; he's also going to carry out revenge upon a society which disrespects his race.
Act 3 is also crucial in the development not just of the play, but of Othello's character. Prior to this moment he's been a thoroughly honest man. But since Iago has been sowing seeds of doubt for so long, he's now starting to lie and conceal his true feelings. He claims not to be coming under the spell of the "green-eyed monster" but it is as clear to us as it is to Iago that his jealousy is beginning to take root deep within his tortured soul. The explosion of hatred and anger cannot be far away:

"Arise, black vengeance, from the hollow hell!
Yield up, O love, thy crown and hearted throne
To tyrannous hate! Swell, bosom, with thy fraught,
For ’tis of aspics' tongues!"

Note that Othello has begun to develop Iago's penchant for animal imagery: "aspics' tongues."
 
Yet still Iago retains his outward show of moderation. But he's achieved his purpose. Not only has Othello become consumed by jealous rage, he's now openly expressing Iago's true thoughts, albeit with much less subtlety. Having been gradually stripped of his manhood, his dignity, and his fundamental honesty, he is now Iago's plaything. Othello and Iago are now united in a common purpose. Crucially, however, it is Othello's langauge that has changed, not Iago's, and that shows us clearly who's really in charge.