Do I need to report on Scope 3 emissions under SECR?

Reporting on Scope 3 emissions is voluntary for quoted companies, but strongly encouraged.

For large unquoted companies and LLPs, some Scope 3 emissions are mandatory, and some are not. You must disclose energy use and related emissions from business travel in rental cars or employee-owned vehicles where they are responsible for purchasing the fuel.  Reporting other Scope 3 emissions is voluntary, but strongly encouraged where this is a material source of emissions.

Reporting on Scope 1 and 2 emissions is mandatory for all companies affected by SECR (quoted companies must report on global emissions, large unquoted companies/LLPs must report on UK emissions).