First of all, head back to Rose Town. Talk to Gaz and a short conversation will start where Gaz shows his mother that Geno actually is real, and where Geno explains Gaz that he needs to collect all the Star Pieces. Gaz gives you the Finger Shot along, which is a weapon for Geno (you know the drill, equip it). Now, head to Moleville.
Moleville is a town consisting of... moles. First of all, head to the item shop to the left. The Mid Mushroom and the Maple Syrup are better versions of the Mushroom and Honey Syrup, restoring 80 HP and 40 FP respectively. You should have already got the Finger Shot from Gaz, so just get the Punch Glove and the Cymbals, which are new weapons for Mario and Mallow. In terms of armor, buy the Mega Shirt and Mega Pants, and two Work Pants (you'll see why later). Equip one to Geno, and equip everything else you got.
Now, head to the far back of Moleville. A cutscene will show Bowser, whose morale has fallen since the last time, with a greatly reduced amount of troops. Afterwards, save in the inn if you want, then head into the mine.
A cutscene will immediately start where some moles talk about how high the entrance is and they wished Mario was here. Well it so happens that Mario actually is here, so agree to rescue the two children and jump up to the entrance.
In the next area there will be some new monsters you need to look out for. Especially the Cluster is dangerous, because it has strong magic and can also self-destruct, which is an instant kill attack. The Enigma is also a problem because it has quite a lot of HP and is hard to take down, but it is weak to Jump so use that. In this area, simply go through, then jump down and go through the passageway. Here, you will see a springboard, so jump up...
...except there's nothing above, so you'll just crash against the ceiling and become unconscious... while Croco steals all your coins. Follow him to the next area. Here he will use a bomb to blast open a new passageway. From now on, the way just leads around - your job is to catch up to Croco while he's running away to fight against him. You can, if you want, fight against the Crooks along the way, they'll give you Flower Tabs if you do. In any case, once you caught Croco, you'll enter a fight with him.
Miniboss: Croco (2)
This time, using Fire Orb does not work too well. Therefore, you should simply physically attack him. He will attack with bombs like last time, so heal frequently especially in the beginning using Mushrooms. Be sure you have enough Flower Points or else you may die. Halfway through the battle, he will steal all your items. You must now fight on your own, and use Mallow's HP Rain when a party member is low on health. He will also start to throw enemies at you, which can be blocked easily. Eventually, you will succeed (750 hp).
Coal Mines, part 2
You'll get a Bambino Bomb Croco left behind. Now, go and follow the train tracks. Eventually, you'll come across another mole. He will take your bomb to blast open the wall, allowing you to go through. In the next area, climb up and use the upper passageway. There, you'll find an Invincibility Star. Quickly run forward through the Bob-ombs, go through the passageway, climb up the stairs, go through, and get rid of the formation of four Bob-ombs before the star runs out.
Behind the formation is a save point, and a treasure box with a Recovery Mushroom. Save, then follow the train tracks. There is another Treasure Box with a flower inside so be sure to collect it. Go farther until you see the next miniboss. You can't approach him from the front so run into him from the side to start the next battle.
This is an annoying battle. Generally, you will want to physically attack him, as special attacks don't work too well here. He will bring a line of bombs into the battle so use Mallow's Thunderbolt to take them all out. They'll start small but become larger as the battle progresses, but they can be easily blocked. Also watch out for Punchinello's Sand Storm attack, as it causes Fear status, making the entire party weaker. If you got Able Juices during previous battles now is the time to use them. You will probably need to heal multiple times during the battle, but hopefully you'll manage to go through(1200 hp).
At the end, he will be angry and try to drop a giant bomb. Eventually it works, but the bomb crushes Punchinello beneath it. It then ignites and explodes. After the explosion, the star piece slips through and Mario gets the third Star Piece!