The three national television networks decided yesterday not to carry live the debate this Saturday night in Iowa of the Republican candidates for president.

CBS said it will televise the debate, but starting at 11:30 p.m. EST, a two-hour delay. NBC and ABC said they will not televise the debate and will cover it only as a "news event," meaning they will broadcast whatever segments they feel generate news.

A spokesman for CBS explained the network decided against live coverage of the Republican debate when President Carter last week pulled out of the Democratic debate with two other candidates, Sen. Edward M. Kennedy (D-Mass.) and California Gov. Edmund G. (Jerry) Brown Jr.

"It appears now there will be no Democratic debate in Iowa," the CBSspokesman said, " and so we felt the Republicans did not warrant prime time in their case. We also felt the Republican debate lost a little when former governor (Ronald) Regan decided not to be involved in the debate."

Both Reagan and Carter last week pulled out of the debates, which dampened the networks' enthusiasm for the Republican debate and killed it for the Democratic set-to, which was due to take place this coming Monday.

In a telegram to the Des Moines Register and Tribune, which was sponsoring the debates, White House press secretary Jody Powell again rejected pleas that Carter reconsider and join in debate with Kennedy and Brown on Jan. 17 in Washington instead of Des Moines.

"The president does not feel it would be appropriate to participate in a partisan campaign debate," Powell said in the telegram to Register Editor James Gannon, " as long as Americans continue to be held hostage in our embassy in Tehran. Until the situation in Iran is resolved, we feel we have no choice other than to reluctantly regret we cannot accept the offer" to debate.

The Register proposed yesterday delaying the debate for 10 days, moving it to Washington and excluding any reference in the debate to the American hostages in Iran.

Brown spokesman Tom Quinn said he was "astounded" by Carter's refusal, saying it "proves he intends to run a closed campaign where the president will be carefully packaged by his advertising people but shielded from tough questioning." Kennedy press secretary Tom Southwick said the senator would stand on the decision he took last week not to debate Brown alone.

Republican candidate John B. Connally said through a spokesman, "His real purpose for avoiding the debates should now be obvious. Clearly, it's not concern for the hostages and foreign policy, but concern over polls and domestic politics."

The Democratic debate was scheduled two weeks before the Iowa precinct caucuses, which are set for Jan. 21. Carter's decision not to participate almost surely means there will be no debate in Iowa.

At the same time the debate in Iowa was losing importance, the CBS-Mutual network station in Miami invited Carter, Kennedy and Brown to a debate Feb. 16 in the Orange Bowl before Florida's primary election in March.

The White House said it had not had time to consider a Florida debate, Brown's campaign headquarters in Boston said it had not received the invitation and Kennedy's campaign headquarters in Washington said it had made no decision on whether to accept the invitation.