How do you battle the 3rd gym leader I go into the gym building and the..

Um, Fantina, that woman you saw, is the 5th gym leader. You have to go to Veilstone, and in that town is the 3rd gym leader. And yes, the gym in Pastoria is the 4th gym. It is a little confusing, I must say, but it's the way the game works, I guess.

So get up to 4 badges, and in Pastoria, a galactic grunt will be leaving to go to Lake Valor to the east. Follow him, and you'll eventually battle your rival. And THEN he'll tell you to follow the guy. So follow the grunt, and when he tires out, he'll battle you. And then, Cynthia will come along, and give you the secretpotion, which heals the psyduck along route 210. So, go to route 210, and near where that moomoo restaurant is, there will be a group of psyduck, that you hopefully noticed earlier on in the game. So go up to them, and talk to them, and they'll be healed. But wait, here comes Cynthia again. She'll talk to you, and then give you the old charms, which is to be given to her grandmother in Celestic. Guess where you're going? How convenient, too! Celestic is just beyond route 210, imagine that. But before you go on, you should have these HM's: HM01 cut, HM02 fly, HM05 defog, and HM06 rock smash. With these HM's, go along route 210, and use defog in the northern part. Now seeing clearly, go through the rest of route 210, and go to Celestic. Go to the center of the town, healing first, and talk through the old woman's converstaion. Battle the grunt, beat him, and talk to the old lady again. She'll notice the old charms, and she realizes that you got them from Cynthia, her grandaughter. So, you give them to her, and now you have to go in the cave and examine the drawing on the back wall. The old lady will come up to you again, and give you HM03. Leave the cave, and go through Cyrus' conversation. With all this done, the 5th gym leader is now accessible!

The third gym leader is in Veilstone. You're thinking of the third leader.

Shes not the 3rd gym leader. Shes the 5th

I believe she is a later gym leader, maybe sixth or fifth, I don't remember, but she is later in the game.

Here we go again.

After you've gotten the 4th gym badge, talk to the Galactic Grunt in Pastoria. He should run off. Chase him, and eventually, you'll find (insert Rival's name). If you had any trouble with him, heal at the Pokemon Center. Then, keep chasing him, until you get to Valor Lakefront. Now, the Valor Lakefront is somewhat of a little town. A little east of the restaurant, is where he'll be. Finally, he'll get the nerve to battle you. All he has is a measly Glameow, so he shouldn't be too much trouble for you right now. After beating him, go north and Cynthia should appear. After spouting her useless blabber, she should give you the Secret Potion. Give them to the Psyducks near Solaceon Town. Then, complete that route, and you should be in Celestic Town. Find a certain old lady in there, and she will spout even more useless blabber. Then, when you're ready battle the Galactic Grunt. He shouldn't be too hard for you at your level. Then, the old lady from before should come back to you, and tell you to check out the cave in there. Do so. She'll then give you HM03, Surf. Finally, talk to the man you saw in Mt. Coronet. Finally, the gym should be open. Phew, that was a lot of typing!

I can't actually help you, but I remember what she said. She said she was going to go train somewhere and I can't find her either!