From StrategyWiki, the free strategy guide and walkthrough wiki

Again, the third world is the grass land.

[edit] World 3-1

• Pidgit returns and is by the waterfall. In the first cave is a warp pipe to World 5. Two Pansers (gray and pink) first appear by the clouds and platforms joining Red Panser and two Shy Guys. The boss in the cave is Red Birdo.

[edit] World 3-2

• Outside, Ostros debut, while Shy Guys, Snifits, Beezos, and Gray Panser return. In the underground level are three Tweeters, one Shy Guy, and four Porcupos. Porcupo debuts. The boss in the first and last underground room is Red Birdo.

[edit] World 3-3

• Ostros and Shy Guys return, and Albatoss and Bob-ombs first appear. In the building are Shy Guys, Ninji, Snifits, Tweeters, two Red Pansers, and Sparks. In the second outside level are five Ninji. The boss is Mouser, who returns from World 1.