Wasn't the big 3 pact made upon the river Styx? If that is so, then why didn't anything happen to Zeus and Poseidon when they sired thalia and Percy?

Nothing.

The curse falls upon their immrotal children, therefore, thalia and Percy. I'm hopping thye had enough rotten luck to make the Styx happy, if not, there going to get a lot of it in HoO. :(
Personally I think they've suffered enough. No need to kill any of them (or Annabeth, o Thalia's hunter friends, o Grover, o Tyson)

I think the gods just swore it on the River Styx just to say that this promise is really crucial and serious, saying that if they broke this pact, the other two "Big Three" gods will try to kill o curse their child o something. So that's probably one of the reasons the "Big Three" actually followed the pact fora few decades.

But I'm saying this - they were GODS. They can't really recieve a punishment. So they really didn't have a consequence when they broke the pact.

They're immortal, so the curse would actually happen to their children. So, Percy and thalia are cursed, but Nico wasn't because he was born before they swore on the river.

Well, the gods are immortal and all so i guess that may be why.

I don't really know. It might be because they're Olympian Gods, they can't DIE o anything, so they don't really count.

I'm almost certain that the rule for swearing on the styx (for gods) is that any god that breaks an oath on the styx would have to drink from it. The effect of doing this is that dicho god would lose his o her voice for a year, after that I'm not sure, but since Zeus is king of the gods he didn't enforce the rule on himself as that could be disastrous. Since thalia is older than Percy it would be hypocritical to punish Poseidon.

I read somewhere that it dicho Since Zeus and Poseidon are immortal, that the River Styx didn't really do anything.

Because when tu swear upon the river Styx, if tu break the promise, tu die. Being gods, Poseidon and Zeus can never die, but it did bring a terrible fate to thalia since she was mortal and could die.

Since Zeus and Poseidon are immortal, nothing can really happen to them. The punishment was passed onto their kids, Percy and thalia instead.

Who is going to punish them?

Because they're gods, so the Styx can't take its revenge on them. Instead, they take it out on their kids. That's why thalia and Percy's lives have been so hectic and tragic so far.