Three countries joined the European Communities on 1 January 1973
On 1 January 1973 three countries joined the European Communities, the bodies from which the European Union descends. It was the first enlargement, and it took the membership from six to nine. The accession treaty had been signed in Brussels in January 1972. Two of the three were founder members of the European Free Trade Association and left it to join. Two of the three are still EU members; the third left the EU on 31 January 2020. The list is fixed: nothing that happens now can add to it or take from it.
Name all three.