Phileas Fogg
I laughed here when I read about the accountant! Yes, it is true. At one point a few years ago SAS had all three types in service. Now they have retired the MD-80 in favour of the Airbus.
SAS was a very faithful McDonnall-Douglas customer and once operated the world's biggest fleet of DC-9's. The merge with Boeing meant SAS could get favourable deals on the 737 although they had started to buy Airbus aircraft already in the 80's to replace even older aircrafts. The plan is to phase out the 737's in favour of Airbus, complete around 2020.
At the moment though, SAS have both 737's and 320's around. Through codeshare with Blue1, there are also B717's in operation (so you could argue that they still have all three aircrafts, LOL!). Blue1 will acquire the 737's and the SAS fleet will be harmonized around Airbus aircrafts.
In the golden days of the 1970's SAS also had DC-10's and some 747's swooshing around.