Will a detached single-story commercial unit of approximately 2400 sq ft split into 3 units each about 800 sq feet need an individual fire risk assessment for each tenant, or can the building as a whole be done in one assessment? The building is owned by one person.

Each unit would be responsible for it's own FRAs  - if they have their own individual fire alarm systems, exits and 60 minute fire resistant party walls separating them then there would be no need for a landlord common FRA covering the whole building.

FRA requirements are based on who is the Responsible Person or Persons, starting with employers, then persons having control and finally if any areas not covered by either of those only then the landlord. So for your scenario there will be three employers, three premises, three FRAs

many thanks anthony.