BHD is the old Belfast Harbour Airport - renamed to George Best.
BFS is the Belfast International Airport code
BST is the city airport - in USA.
Codes are generally made from letters in the name of the destination, and as there are 33 000 codes, sometimes it is not possible to use the obvious combinations as airports locations are all over the world in places with similare names so there is a chance the most relevant codes have been allocated.
There is also a 4 letter code for airports used by the control towers round the world which is not the same as the IATA travel operators code. BHD for flight planning is EGAC. This is made up from locating in a part of the world then in what country then if in a group and then the city.
and pilots will also usea 5 letter code too....
Now for a bobble hat prize.....