I'm not sure any questions can fully clarify the ethical nature of a politician since I am convinced that very few of today's politicians are honest or ethical. Lies and misinformation are among a politician's greatest weapons in today's world, and I wouldn't trust the word of a politician any farther than I could throw him.

A political action is ethical if it normally would be ethical.  For example, if you consider it unethical to bribe someone, then it is unethical to take a bribe as a politician.  People’s ethics to matter to them, and they do decide who to vote for based on whether or not the politician acted ethically. 

Politicians routinely resign due to affairs or other unethical behavior that is not directly related to their job, because people assume if you cheat in one area of your life, you will cheat in others. 

"There's good evidence that men with great power and status are, in fact, more prone to affairs," says Todd Shackelford, who directs the evolutionary psychology lab at Oakland University in Michigan. (npr)

The bottom line is, if it seems shaky you should not accept it!  Ethics may be personal to a certain extent, but right and wrong does matter in politics.

Since all politicans swear oaths to uphold the Constitution or the laws, one ethical question to ask would be whether one's actions violate that oath. Or, perhaps, if violating that oath might sometimes be ethical in a situation where lives were at stake, for example. Politicians might also think about whether their actions are consistent with the spirit of the laws or a nation's history. 

Some other ideas include:

• What is the intended outcome of the decision?
• Do you have any regrets about the decision? (This question would be used in hindsight, or rephrased as: "Might you have any regrets about this decision?")
• Is this action legal?
• Will anyone be hurt by this decision? Who and how?

Another question that might be asked is whether or not the politician would be comfortable reading of his or her activities as a headline in the newspaper.  This is a question that makes a person think twice! 

One question we might ask is, "Who stands to gain from this action?" and another is "What does the politician stand to gain from this action?" 

If a public official stands to gain something while the public stands to gain little or nothing, we might infer that this political action is unethical (motivated by self-interest). 

I do not really think that there are exactly three questions that everyone would agree can be used to determine whether a political action is ethical.  It may be that your text specifies three of them, but I think that there are many possible questions that could be asked.

For example, we could ask “are more people helped than hurt by this decision?”  We could also ask “are the people who are most vulnerable in society helped or hurt by this decision?”  As another alternative, we could ask whether the person taking the action was going to profit tangibly from the decision. 

These are some possible questions, but there are surely others.