The nine-nation European Economic Community is now reaching a crucial phase in the controversial process of extending its membership to the three Southern European countries -- Greece, Portugal and Spain -- which have been actively seeking to join the rich man's club to the North ever since their enfranchisement from dictatorial rule earlier this decade.

Spurred by the EEC's political determination to consolidate West European democracy, negotiations fro Greek membership are scheduled for completion by year-end, while substantive talks with first Portugal then Spain seem likely to be highlights of European diplomatic activity in 1979 By the early 1980s, predict experts here, the prospect of a 12-member 300 million strong European community, stretching from the Eastern Mediterranean to France's Atlantic seaboartd in the West, and from the Baltic Sea in the North to Gibraltar in the South, should be hard political reality.

For there is no doubt now that the EEC has proclaimed political commitment to open its doors to West Europe's renascent democracies is being severely tested by the economie fears triggered by the addition of rival rather than complementary economies to the existing Common Market.

Anxiety is most acute in France and Italy where farmers in the Eec/'s Mediterranean sunbelt are faced with the prospect of cut-throat competition in particular from cheap Spanish and Portuguese wines and Greek fruit and vegetables.

But France and Italy are not alone in their concern. Even mighty Germany, the most economically resilient of Common Market countries, is clearly worried at the impact of migrant workers from Southern Europe on its national job market.

Faced with these internal strains, the EEC's emerging response seems to be a slowdown in economic integration of the three candidate countries, but offset with the granting of full political membership. The upshot is that the Eec/ could well be a 12-member political unit early in the 1980s but not a fully integrated economic entity until 1990 and beyond.

Application of the economic brakes has already started. Last week Greek ministers came here and, much to their displeasure, were firmly told by EEC negotiators tha they would have to wait 8 years after foining before their workers could move freely throughout the Common Market.

The Madrid government will face an even tougher deal.The EEC's executive commission, aware of Spain's high performance in sectors like steel, textiles, shipbuilding, fisheries and wine growing, called last week for a 10-year period to cushion the competitive impact.

But the growing pains facing the enlarged European community should not hide either its increased political weight nor the new directions in which it might exerxise it.