In Harry Potter and the Chamber of Secrets, Harry, Ron and Hermione turned into three of the Slytherins using Polyjuice Potion. If they could brew such an immensely complicated potion, why didn't Harry, Ron and Hermione use Veritaserum or a Confundus Charm instead of Polyjuice Potion?
I'm ignoring here how they would be used as the question just assumes they could be.
Out of Universe
It hadn't featured yet. Veritaserum makes its first appearance in Harry Potter and the Goblet of Fire, the Confundus Charm is mentioned in passing in Harry Potter and the Prisoner of Azkaban. It's possible that JK Rowling hadn't even thought of them when she was writing Harry Potter and the Chamber of Secrets.
They hadn't learned them yet, and possibly even hadn't learned of their existence.
Veritaserum is typically taught to students after they've passed their O.W.L. The Confundus Charm was typically taught to fifth year students in preparation for their O.W.L.
In Harry Potter and the Chamber of Secrets, they're in their second year. They simply hadn't learned the spells yet.
Polyjuice Potion, however, has been mentioned by Snape during potions class, which is how Hermione learned of it.
Using Polyjuice potion is much stealthier
While SQBs answer is spot on and lists the most important out- and in-universe reasons, there is another aspect: When using Polyjuice potion, the victim may never become aware he or she was spied on. Even if the deception is discovered, this doesn't mean there is any clue leading to the perpetrators (except cui bono and possibly modus operandi, neither really hard evidence).
Veritaserum may be applied secretly, but the victim will still be aware that he or she was asked, by whom, and which questions -- not really the hallmark of successful undercover work.
The Confundus Charm, on the other hand, can be used to make the victim spill the beans and then forget the whole affair, but that takes considerably more skill than the simpler applications of the charm. In particular, I deem it difficult to coax reasonable statements out of a confused victim; "Millenium hand and shrimp!" is more like what I would expect to hear. Moreover, any failure happens directly in front of the victim -- a sudden end for the stealthy approach. In contrast, Polyjuice potion takes no real skill to use, and the one failure with it (you may remember Catgirl Hermione) happened away from the action and did not impact the success of the surreptitious questioning in any way.
Both Veritaserum and Confundus are highly intrusive and violate the integrity of the mind of the victim. This really doesn't fit the tone of the novel nor the characters of supposed heroes. Their use might become justifiable in the later, increasingly violent novels, but not in the second one, which is still more like a boarding school novel where the children play tricks on each other.