UK Building Regulations: which document is regulation 3 in?
I can't find regulation 3.


Follow this trail from http://www.planningportal.gov.uk/buildingregulations/


Building Policy and Legislation
Current Legislation
Building Regulations
Building Regs Legislation 2010
The Building Regulations 2010 (SI 2010/2214)  
Part 2,
3. Meaning of Building Work


Which leads you to The Building Regulations 2010, No. 2214, PART 2, Regulation 3

Meaning of building work
  3.—(1) In these Regulations “building work” means—  
       (a) the erection or extension of a building;  
       (b) the provision or extension of a controlled service or fitting in 
           or in connection with a building;  
       (c) the material alteration of a building, or a controlled service or
           fitting, as mentioned in paragraph (2);  
       (d) work required by regulation 6 (requirements relating to material 
           change of use);  
       (e) the insertion of insulating material into the cavity wall of a 
           building;  
       (f) work involving the underpinning of a building;  
       (g) work required by regulation 22 (requirements relating to a change 
           of energy status);  
       (h) work required by regulation 23 (requirements relating to thermal 
           elements);  
       (i) work required by regulation 28 (consequential improvements to 
           energy performance).  
    (2) An alteration is material for the purposes of these Regulations if 
        the work, or any part of it, would at any stage result—  
       (a) in a building or controlled service or fitting not complying with 
           a relevant requirement where previously it did; or  
       (b) in a building or controlled service or fitting which before the 
           work commenced did not comply with a relevant requirement, being 
           more unsatisfactory in relation to such a requirement.  
    (3) In paragraph (2) “relevant requirement” means any of the following 
        applicable requirements of Schedule 1, namely—  
           Part A (structure)   
           paragraph B1 (means of warning and escape)   
           paragraph B3 (internal fire spread—structure)   
           paragraph B4 (external fire spread)   
           paragraph B5 (access and facilities for the fire service)   
           Part M (access to and use of buildings).