All three terms refer to different types of sustainable investment and can be distinguished in the following ways:
ESG investing aims to create long-term wealth by focusing on core environmental, social and governance criteria. This is based on the premise that companies employing such focused and sustainable business practices tend to generate better financial returns.
Impact investing intends to deliver positive social and environmental impact alongside financial returns by investing in businesses and organisations with a positive social mission.
Socially responsible investment (SRI) is sometimes used as a broader term, which encompasses ESG and/or impact investing. However, it can also narrowly refer to an ethical investment strategy that uses defined criteria to exclude potentially damaging social and environmental investments from a given fund (e.g. tobacco, alcohol, firearms, fossil fuels) and is widely available in public markets.