President-elect Jimmy Carter's proposed tax and spending program is probably too modest to enable him to reach his gaols of lowering the unemployment rate to 6.5 per cent by years end and boosting the econimic growth rate to 6 per cent, three leading economists testified yesterday.

None of the economists - including director of the Congressional Budget Office and two formet chairmen of the Council of Economic Advisers - called the program inadequate, but each implied that the amount of economic stimulus it contined was too little.

An Analysis of a set of proposals similar to Carter's by Congressional Budget Office director Alice Rivlin indicated the President-elect could reach his targets only if every "optimistic" assumption in her forecast came true.

And noth economic advisers - Paul McCracken, who headed President Nixon's council, and Walter W. Heller, who headed the Kennedy-Johnson panel - called for programs bigger than the one proposal late last week by Carter.

Their testimony came before the Senate Budget Committee which is considering whether to revise the budget it adopted last September because economy is weaker than it was predicted to be.

The Carter package would spen between $23 billion and $30 billion over the next two years, and probably less than $15 billion a year.

McCracken, a conservative economist who is a professor at the University of Michigan, said he wuldd devise a package as big as $24 billion a year. Heller, a liberal who is a professor at the University of Minnesota, kidded that he was "just a little more cautious than my radical friend (McCracken)" and suggested a $20 billion program. Nevertheless, Heller said, "I wouldn't serously object to $24 billion."

Carter reportedly relied heavily on his designate for director of the Office of Management and Budget, Thomas B. (Bert) Lance, who argued that a package bigger than $15 billion would enlarge the federal deficit too much and lose some business community confidence.

Last week Carter proposal a one-shot tax rebate for individuals between $7 billion and $11 billion, permanent tax relief for individuals totalling $4 billion and a business tax deduction based on the size of its payroll tax deductions totalling $1 billion in fiscal 1977 (which began Oct. 1) and $2 billion a year after that.

Carter also proposed increased government expenditures amounting to as much as $2 billion in this fiscal year and between $5 billion and $8 billion in the federal spending year which begins Oct. 1.

The Congressional Budget Office analysis of a package which Rivlin said seems similar to the Carter proposals would reduce the unemployment rate by about 0.45 percentage points below where it otherwise would have been a range from between 6.6 per cent to 7.4 per cent.

Economic growth would be raised to a range between 4.5 per cent and 6 per cent.

Rivlin said budget office provides a wide range of polsibilities because of the great incertainties in forecasting economic behavior. However, on the basis of the analysis she presented, for the Carter program to meet its goals, unemployment and growth would have to perform at their most optimistic levels.

Under questioning, she said the Carter program assumes that federal spending programs will be able to disburse funds and hire people faster than the budget office analysis, but maintained that if the "optimistic end" of the budget office projections is correct, Carter would see his goals met.

She said the inflation risks of the package are small, adding about 0.3 to 0.5 per cent to the inflation rate by 1980. McCracken and Heller also agreed that the inflationary risks of economic stimulus are not that great.

Heller termed the Carter proposals "realistic and progmatic to the point of being downrigh modest." He noted they add up to less than 1 per cent of the protected $1.870 billion gross national product projected for 1977.

McCracken said that most of the tax cuts should be permanent - to have a continuing rather than one-shot impact on the economy - and opted for $12 billion in personal tax cuts, $16 billion in business tax cuts and the rest in public works spending.