The correct answer is option 3: Bug. Bugs are insects that belong to the class Insecta. Insects have three body segments: head, thorax, and abdomen, and typically have three pairs of jointed legs. This means that bugs, unlike spiders, scorpions, and mites, have six legs in total.

Option 1: Spider - Spiders are arachnids, not insects. They have four pairs of legs, meaning they have eight legs in total, not three pairs.

Option 2: Scorpion - Scorpions are another type of arachnid, not insects. Like spiders, they also have four pairs of legs, totaling eight legs, not three pairs.

Option 4: Mite - Mites are tiny arthropods that can be found in various habitats. They may have different numbers of legs depending on the species, but most mites have fewer than six legs, and they do not typically have three pairs of legs.

Therefore, the only animal among the given options that has three pairs of legs, or six legs in total, is the bug.