A 
3-piece basin waste seal kit is a set of specific components designed to provide a secure, watertight seal for a standard 1¼" (32mm) bathroom basin waste fitting without the need for additional silicone, putty, or mastic. 
Kit Components
The three parts of the kit typically work together to create a reliable, long-lasting seal: 
Tapered rubber seal: This large, flexible component is pushed into the basin aperture from underneath. It compresses as the nut is tightened, forming the primary watertight seal between the waste fitting and the basin itself.
Foam or plastic washer: This provides an even surface for the locking nut to press against, ensuring uniform compression of the rubber seal.
Waste lock nut (retaining nut): This plastic nut is tightened onto the threaded portion of the waste fitting from underneath the basin, securely holding all the components in place and creating the tight seal. 
Benefits & Usage
Ease of installation: Reviewers often praise these kits for simplifying the process of installing or repairing a basin waste, especially in traditionally difficult-to-seal areas.
No extra sealants: The kit's design makes using messy and time-consuming sealants like silicone or plumber's putty unnecessary.
Compatibility: They are designed to be universally compatible with most standard 1¼" (32mm) UK bathroom basin wastes, including slotted and unslotted types.