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In order to truly understand these verses, I think it would be helpful to print them out in the diatribe fashion that they were meant to be read.

Objection 1
1 Then what advantage has the Jew? Or what is the benefit of circumcision?
Response 1
2 Great in every respect. First of all, that they were entrusted with the oracles of God.

Objection 2
3 What then? If some did not believe, their unbelief will not nullify the faithfulness of God, will it?
Response 2
4 May it never be! Rather, let God be found true, though every man be found a liar, as it is written, THAT YOU MAY BE JUSTIFIED IN YOUR WORDS, AND PREVAIL WHEN YOU ARE JUDGED."

Objection 3
5 But if our unrighteousness demonstrates the righteousness of God, what shall we say? The God who inflicts wrath is not unrighteous, is He? (I am speaking in human terms.)
Response 3
6 May it never be! For otherwise, how will God judge the world?

Objection 4
7 But if through my lie the truth of God abounded to His glory, why am I also still being judged as a sinner? 8And why not say (as we are slanderously reported and as some claim that we say), "Let us do evil that good may come"?
Response 4
8 And why not say (as we are slanderously reported and as some claim that we say), "Let us do evil that good may come"?

Objection 5
9 What then? Are we better than they?
Response 5
Not at all; for we have already charged that both Jews and Greeks are all under sin; 10 as it is written,
"THERE IS NONE RIGHTEOUS, NOT EVEN ONE;
11 THERE IS NONE WHO UNDERSTANDS,
THERE IS NONE WHO SEEKS FOR GOD;
12 ALL HAVE TURNED ASIDE, TOGETHER THEY HAVE BECOME USELESS;
THERE IS NONE WHO DOES GOOD,
THERE IS NOT EVEN ONE."
13 "THEIR THROAT IS AN OPEN GRAVE,
WITH THEIR TONGUES THEY KEEP DECEIVING,"
"THE POISON OF ASPS IS UNDER THEIR LIPS";
14 "WHOSE MOUTH IS FULL OF CURSING AND BITTERNESS";
15 "THEIR FEET ARE SWIFT TO SHED BLOOD,
16 DESTRUCTION AND MISERY ARE IN THEIR PATHS,
17 AND THE PATH OF PEACE THEY HAVE NOT KNOWN."
18" THERE IS NO FEAR OF GOD BEFORE THEIR EYES."

19 Now we know that whatever the Law says, it speaks to those who are under the Law, so that every mouth may be closed and all the world may become accountable to God;

20 because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin.

With that fashion of reading in mind, let's dive in.

As shown above, Paul continues with the diatribe style, answering imaginary questions. The Jews reading this would naturally be concerned about their "status before God." Paul answers this question in two different ways in 3:2 and 3:9. The first in 3:2 is saying that the Jews have great privileges. Humorously, Paul says "first of all," and then never truly returns to the list, getting too caught up in the diatribe. However, this thread of thinking is eventually picked up again in 9:4-5.

Anyway, the "first of all" is simply that the Jews were entrusted with the oracles of God. But immediately the Objector asks if the unbelieving will undo God's faithfulness, a very legitimate question since the Jewish nation had been utterly unbelieving. Paul of course answers that God will do what he wants.

The Objector then asks the questions with a mindset of "the end justifies the means." Essentially the Objector asks in 2 different ways (3:5, 7-8) that if being bad makes God look good, then why does God care if we are being bad. Paul's objections may seem weak, (3:6 "May it never be," and 3:8b "Their condemnation is just.") but he simply refused to give ear to such blatant self-refuting arguments. God looks good no matte if people are doing bad things or not.