Are there requirements for periodic inspection of your consumer unit? (UK)
There is not a requirement in owner-occupied single family dwellings to have periodic electrical inspections. There is in some rented homes or houses of multiple occupation (HMO). 

Under BS7671 (the IET) Wiring Regulations the electrician is required to recommend an inspection period. I don't know why yours has put 3 years, for dwellings the interval is usually 5 for rented and 10 for owned.


  For rented accommodation, the maximum period recommended between the
  initial inspection (when the installation was first put into service)
  and the first periodic inspection and test is five years.


NICEIC recommendation


  change of occupancy/10 years


Intersafe electrical test providers


  Typical workplaces or environments which require 3 or 1 year Fixed
  Wire Testing include: Spa hotels and leisure centres – 3 years


Intersafe electrical test providers 2

If you have the previous test certificate or can get a copy from the electrician who issued it, there may be an explanation for why a more frequent inspection has been recommended, eg there may have been previous signs of "green goo" in the installation.