There are several Harness systems on the market that are not FIA Homologated but are ECE approved to be legal for road use.
For disciplines such as the Road Car Categories in Sprint and Hill Climb, where the minimum harness requirement is a 3-point belt per NCR Ch.14 App.4 Art.2.6. Providing the harness used bears all the correct ECE approvals and markings to be legal for road use and is installed correctly in a way that is fully compliant with MOT standards for seat belt mountings then this type of harness would be acceptable.
However, in no way should this be considered as replacement for a 4-point harness, where the NCR requires 4-point as a minimum, nor would it be acceptable where there is a requirement for an FIA homologated harness.