Ge 1:11 Then God said, "Let the earth bring forth grass, the herb that yields seed, and the fruit tree that yields fruit according to its kind, whose seed is in itself, on the earth"; and it was so.

12 And the earth brought forth grass, the herb that yields seed according to its kind, and the tree that yields fruit, whose seed is in itself according to its kind. And God saw that it was good.

Why did Moses mention only three types of plants here?

 God did create all others of course. But Moses is stressing only the plants that were primarily used by Israel as food for man and animals during their time. He wants to show that the vegetation that sustained man came from God. He created them. It was not some other deity that did it.