In less than three months, Greece will become the 10th member of the European Common Market, a long-awaited move that is expected to have profound effects on the country's economic, social and political structures.

Greece's membership, which takes effect Jan. 1, also constitutes the first enlargement of the European Community since Britain, Ireland and Denmark joined in 1973.

For the Common Market, Greek accession represents a first step in a southward expansion into the Mediterranean that eventually will include Spain and Portugal, a process that is thought to have important political benefits although it constitutes a risk largely because its new southern wing will have a substantially lower level of economic development.

For the Greek government, full membership in the European Community after 18 years of association with it is considered a major political victory, the economic aspects of which are expected to be largely beneficial.

Greece's decision to establish links with the community dates back to the late 1950s when the government opted for an association treaty with the Common Market in lieu of full membership in EFTA, the European Free Trade Association. When the Greek military junta collapsed in 1974, then-prime minister Constantine Karamanlis decided to speed up the process and to become a member state of the Common Market.

Although Greek government officials have always insisted that membership would provide Greece with a welcome economic stimulant, there is widespread recognition that when Greece emerged from seven years of dictatorship in 1974, Karamanlis' motive was primarily a political one.

"Greece could not absent herself from this common effort . . . because its future is closely; I would say inseparably, linked with that of the other democracies of Europe," he said in May 1979 when the treaty of accession was finally signed.

According to one Common Market official, "Karamanlis was afraid of the extremists among his countrymen and sought to bind Greece to the West as closely as possible."

"There may be some adjustment difficulties, but this is where we belong," a high-ranking Foreign Ministry official said last month. He explained that the country's political leaders "had wanted to make sure that Greece would be an integral part of an integral politically democratic Europe" that eventually would become the nucleus for a European federation.

But Karamanlis' desire to link his politically fragile country to the democratic West may have led Greek officials to play down the problems of adaptation that full membership is expected to bring.

"There will be no dramatic changes," says George Kontogeorgis, the Greek minister for European Community affairs. Kontogeorgis, who was recently appointed Greece's first Common Market commissioner, believes that inflation has already brought Greek agricultural prices close enough to European levels to avoid sharp cost-of-living increases for Greek consumers.

Kontogeorgis also believes that after almost two decades of association, the customs union with the Common Market has been substantially achieved and that the five-year transition period provided for in the treaty will make removing a remaining 32 percent of customs duties relatively painless.

"Of course there will be difficulties," he says, "but in this way we will be forced to swim in deep water."

Others, however, do not share this optimistic outlook. Although charges by left-wing groups that Common Market membership will cause uncontrolled price increases, widespread unemployment and a crippling invasion of foreign capital are generally considered exaggerated and alarmist, there will be some substantial costs.

"Those Greeks who believe it's going to be all gravy are going to be in for a surprise," said one Western diplomat. "It will be a hard test of the country's readiness."

He pointed out that the country's outdated banking system, its long dependence on protectionist measures and exchange controls -- the Greek currency is not now fully convertible -- and its fragmented and antiquated agricultural sector will combine to make adjustment to the European Community a painful, although ultimately rewarding, process.

"The Greek economy is not really ready for membership," says Leonidas Kirkos, a pro-Common Market, leftwing politician, "and much restructuring in both agriculture and industry is going to be needed."

"There definitely will be traumas," John Piperoglou, general secretary of the Federation of Greece Industries. He predicts that many of the traditionally small Greek companies will probably disappear or combine with bigger European companies.

"There are major opportunities for foreign firms to buy into Greek firms," he added.

Greece's accession to the Common Market may also be made more difficult by current economic problems, largely 25 percent inflation, and a balance-of-trade deficit that is expected to produce a 1980 current account deficit of as high as $2.5 billion.

Indeed, after three decades of steady growth that for most of the period following the civil war averaged between 5 percent and 7 percent a year, the economy of this small Mediterranean country has entered a period of stagnation. Partly as a result of more stringent fiscal and credit policies, economists expect 1980 to bring near-zero growth.

The prospect of enlargement southward has raised fears among some Common Market member states about possible competition from an invasion of Mediterranean produce and an overall loss of momentum, primarily in the development of the internal market and the achievements of economic and monetary union.

But in the case of Greece, Common Market officials appear convinced that because of current disparities in Greek and community infrastructures, living standards and tax systems, not to mention the far greater importance of agriculture to the national economy (the sector employs one third of Greek workers), the main economic implication of Greek membership for the community will probably be budgetary and financial.

As in the case of Spain and Portugal, however, the European Commission is convinced that the consolidation of Greek democracy and the greater international status of an enlarged community will far outweigh the costs.

Athens publisher Helen Vlachos, who believes membership will end decades of isolation for a country that is part of no geographical or linguistic group of nations, said that "anyway you look at it, a poor family is always better off in a rich environment."