Avocadoes drop from Avocado Trees, which can be found in Swamp biomes. They can be eaten as is to restore 2 points of hunger, and have a nourishment value of 1.2, they are also used in the preparation of Guacamole. Avocados are placed in the crafting table to get an Avocadoe seed.
Watermelons can be found in abundance in Swamp biomes, each slice of melon restores 1 point of hunger and has a nourishment value of 0.6. They can be used to make Melon Juice and are an ingredient in Fruit Salad
Yes, it's a fruit :P. Tomatoes are harvested from Tomato bushes, and can be eaten as is to restore 2 points of hunger, or turned into Tomato Juice. They have a nourishment value of 0.6. They are also used in the preparation of Salad. Tomato seeds can be crafted from a tomato.