I assume that '3 years' was a typo for '3 days', as Britain declared war on Germany on 3 September 1939, (just a few weeks after I was born). In one sense, the advert for TV engineers might seem to have been ill-timed, but of course, all radio manufacturers and other engineering firms would soon be working at full tilt manufacturing equipment for the war effort. Hence, many who worked in the industry would have been declared exempt from being called up, though others would have been needed to service and operate communications and radar equipment in the armed services.
On the day that war was declared the UK government enacted the National Service (Armed Forces) Act, which enforced full conscription on all males between 18 and 41 who were residents in the UK. In addition to males aged over 41 there was quite a list of other exemptions, albeit many of those exempt could still be put to work for the war effort and were on the 'home front'.
Exemptions:
•People working in reserved occupations like baking, farming, medicine, and engineering.
•Medically unfit as well as the blind, disabled, and those with mental disorders.
•British subjects from outside Britain and the Isle of Man who had lived in the country for less than two years.
•Students.
•Persons employed by the government of any country of the British Empire except the UK
•Clergy of any denomination.
•Married women.
•Women who had one or more children 14 years old or younger living with them. This included their own children, (legitimate or illegitimate), stepchildren, and adopted children if the child was adopted before 18 December 1941. Pregnant women were liable to be called up but in practice they were not.
•Conscientious objectors
Britain's colonies sent over two and a half million men to fight for Britain during the war - all volunteers. India sent the most soldiers. At that time, India included both Pakistan and Bangladesh. Canada, Australia, New Zealand, South Africa and Rhodesia (now Zimbabwe) also sent countless thousands of soldiers. Hence, Britain had soldiers from five continents:
Europe, North America, Australasia, Asia and Africa.
As to TV broadcasting, on 1 September 1939 at around 12.15pm the service came to an abrupt halt. A Disney cartoon, 'Mickey's Gala Premiere', was the last programme shown before the transmitter was switched off.
https://www.bbc.co.uk/historyofthebb...neral/tvstory9
That said, the time of the closedown is disputed here:
https://www.teletronic.co.uk/pages/h...the_bbc_7.html
TV broadcasting recommenced at 3pm, 7th June 1946, for the select few who had a TV.
Some interesting stuff at this link about Alexandra Palace's 'Secret War':
https://www.teletronic.co.uk/pages/h...the_bbc_8.html
Hope that's of interest and not too far off topic.