Quote 1
POLONIUS
He knew me not at first; he said I
was a fishmonger. He is far gone. And truly, in my
youth, I suffered much extremity for love, very near
this. (2.2.205-208)
Polonius is convinced that Hamlet is "far gone, far gone" in his love for Ophelia. But—sorry, Polonius—we think it's more likely that Hamlet is likely making a naughty joke at Polonius' expense: a "fishmonger" is slang for "pimp." I.e., Hamlet knows that Polonius is basically pimping out his daughter to get at Hamlet.
Quote 2
POLONIUS
This above all: to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man.
(1.3.84-86)
Polonius likes to dish advice, but he sure can't take it. Given Polonius' penchant for spying on his children and Hamlet in order to curry favor with King Claudius, he's not in any position to be talking about truth.
Quote 3
POLONIUS
Marry, sir, here's my drift,
And I believe it is a fetch of wit.
You laying these slight sullies on my son,
As 'twere a thing a little soiled i' th' working,
Mark you, your party in converse, him you would
sound,
Having ever seen in the prenominate crimes
The youth you breathe of guilty, be assured
He closes with you in this consequence:
'Good sir,' or so, or 'friend,' or 'gentleman,'
According to the phrase or the addition
Of man and country—
(2.1.43-54)
What was that about honesty, again? Here, Polonius instructs his servant to spread rumors about his son Laertes in the hopes of finding out what the boy's up to. Obviously, this way of thinking has some major flaws—but this is actually pretty much the same method Hamlet uses to find out whether or not the ghost is telling the truth about Claudius. Hm.