The standard length for A-levels is a two year course, with the option to take an AS exam after the first year for subjects that still provide AS qualifications (as not many do anymore, the only AS at my school is maths).
*What they are asking for is you take ALL your exams in May/June of 2020 (for example) - and also look at your AS exams you took the year before in May/June 2019*
It sound very silly to do your a-levels over three years in all honesty, it doesn't look particularly impressive as the majority of students sit the exams for three or four a-levels all at the same time. This university seems to be requiring the same thing, which isn't surprising as this is the standard expectation that everyone completes their a-levels all together. You can basically do your AS levels after one year of the course, and then your a-levels in the second year, but they do not want you to take three years.
Why do you want to take three years to do fairly easy qualifications?