The 3-inch rear suspension bump stop's primary function is to limit the amount of compression in the suspension system, specifically to prevent the rear suspension from bottoming out.

Compression Limitation: When your suspension compresses fully (like during heavy bumps, off-roading, or hard cornering), the bump stop absorbs the final impact before the suspension components hit their physical limit. This helps prevent damage to the suspension, frame, and other components by acting as a cushion.

Preventing Bottoming Out: If you don't have an adequate bump stop, your suspension might fully compress, causing metal-to-metal contact, which can be harsh on the components and lead to premature wear or even failure.

Ride Quality: Bump stops can help maintain a more controlled ride by smoothing out the harshness of bottoming out. A well-sized bump stop provides a progressive cushion, rather than a sudden hard impact.

Adjustable or Pre-sized: In this case, a 3-inch bump stop usually means it's designed to extend that much into the suspension travel. Depending on the setup, it can be made of rubber, foam, or other materials, and sometimes it's adjustable, allowing for fine-tuning of when and how it engages.