Based on the information from plant cell diagrams and types of plant cells, number 3 in the diagram is most likely the vacuole. In plant cell diagrams, the vacuole is usually shown as a large water-filled component that can occupy a significant part of the cell. Its primary function is to store water, ions, sugars, enzymes, and other substances. The vacuole is enclosed by a membrane called the tonoplast. This matches well with typical labeling and structure of plant cells where the vacuole is a prominent feature marked especially by a larger numbered label, often around the center of the cell diagram.

Therefore, the plant cell part shown by number 3 in the diagram is the vacuole.