President Carter probably will sit out a first presidential debate if both Republican Ronald Reagan and independent John B. Anderson take part, Carter's campaign chairman said yesterday.

The League of Women Voters said Wednesday it plans to go ahead with its first debate Sept. 21 in Baltimore if any two of the three major candidates show up.

Carter has demanded that Anderson stay out of the first debate, and Anderson has refused. Reagan says Anderson must be allowed to participate.

"We have made no change from our basic position that we are going to meet Ronald Reagan head-to-head before we get in any other debate," Carter campaign chairman Robert Strauss said yesterday.

"Once we have done that, we will meet anybody, any place, any sponsor that wants us, in any reasonable terms." Reporters asked Strauss outside the White House whether Carter would refuse to participate in an initial debate if both Reagan and Anderson take part.

"I would think probably so," Strauss replied. "We're better off if we can get Reagan one-on-one. Reagan is better off if he can dilute that."

Reagan told United Press Internatinal earlier he will not debate Carter unless independent Anderson is included, and Reagan said he feels two debates are enough in any case.

"Governor Reagan very wisely wants John Anderson there. It gives him some additional dilution of the debate," said Strauss.