Your calculations are a little off: i.e. if there's already a pot of 3000, and someone bets 1500 (half the pot into it), then you have to pay 1500 in order to win 4500 (the 3000 already in the pot, plus the 1500 bet going into it); in other words, you're getting 3:1 odds which is really good! You only need a 30% chance of hitting a hand to mathematically justify calling this bet. If someone bet 3000 into a 3000 pot (also known as a 'pot sized bet') then your odds are a little worse (paying 3000 to win 6000, or about 2:1), meaning you need at least a 50% to hit your hand in order to call. Remember when calculating pot odds to calculate everything in the pot, including what was in before and what has just been bet.
When it starts to become REALLY useful is when you can begin to put your opponents on a range of
hands. For example, lets say that in your example, given your opponents betting patterns and any
tells (if it's a live game), you think your opponent has a monster hand (i.e. JJ+ or AQ+). If that's the case, then your number of outs changes (namely, a J is no longer an out, but is instead an anti-out, because it could be giving him his set). Or lets say you can tell your opponent is on a flush draw, and because of the preflop action you think he has at least K or A high. In that case, it eliminates all your heart draws because he may have the nut flush, and so on and so forth.
In other words, if you can combine sound pot-odds theory, with skillfull reads and assessments of your opponents hands, then you are already on your way to becoming a long-term successful player. That's why new-breed players like Durrr and Phil Ivey are so hard to play against: they pretty much bet and act the same whether they have pocket AA or 84o, so its very hard to get a read on them, which makes it very hard to play effectively against them no matter how good your pot odds are.
This is also a tool you can use against your opponents when you begin to think about what THEY think that YOU have, and whey THEY think YOU think THEY have. To make this a little clearer... lets say that you decided to play 8s6s from middle position to shake up your game, and raised 3xBB, and the BTN (a very tight player) decides to call you.
The flop miraculously falls 9s7sAh. You have an open ended straight flush draw, and there's also an A on the board, which means that your opponent (playing very tight) may have hit top-pair top-kicker, or even a set if he's slowplaying AA. You decide to fire a c-bet (or continuation bet, following through on your preflop raise), and again, your opponent (a typical Nit) cold-calls you. The turn then comes 5s: you just hit your straight flush. More importantly, your opponent possibly just made the high-flush (if we can also assume he's could playing a top hand like AKs). He however probably thinks you have something like pocket pairs, or at the most a straight, but he will probably not suspect you have the straight flush. Using this knowledge, you can size your bets to draw him into the pot, knowing he doesn't suspect your straight flush, including check-raising.
If you suspect that your opponent is drawing dead (for example in this case he has something like AsKh, drawing one more card to a flush that still loses to your straight flush), you can use pot odds to make it mathematically correct for him to call your bet, even though he can't win. Conversely, if the tables were reversed, if by putting your opponent on a range of hands you know that even though calling to your draw is mathematically correct, your opponent has you beat, knowing pot odds AND being able to read opponents is a lethal combination.
Hope that makes sense... I'm a little bit drunk!