The part labelled 3 is the auxillary bud. This is a structure that is found on the stem of a plant, typically at the base of a leaf or at the node. The auxillary bud has the potential to develop into a new branch or flower cluster. It contains a dormant shoot that can grow when activated by environmental factors or pruning. The auxillary bud is important for the growth and development of the plant and plays a key role in branching and flower production.