House speaker Thomas P. (Tip) O'Neill Jr.'s announcement yesterday that three Republican House memebers had made overtures about switching to the Democratic side of the aisle caused two Republicans to confess nervously they were thinking about it.

O'Neill refused to name the Republicans saying that would be "breaking a confidence." But then he dropped the tantalizing hint that two of them had either 12 or 14 years of service in the House and were negotiating an attempt to keep some of their seniority.

Rep. John Buchanan (R-Ala.), a 49-year-old Protestant minister and moderate Republican, elected in 1964, said, "I have to admit to the possibility that I am one of three." However, Buchanan said he had "strong ties to the Republican party" and added, "I fully expect to remain a Republican for the rest of my career."

And Rep. Charles Whalen, 56, a liberal Republican from Dayton, Ohio, said, "I am not one of the three" but then admitted he might run in 1978 as either an independent or a Democrat, if a state law can be changed.

Whalen said he had not personally spoken to the Speaker but acknowledge a high-level House Democrat had been "courting me." He admitted it was "generally known that I've been concerned about the (conservative) drift of the Party" and admitted two other factors made him a likely candidate.

One was his unhappiness with the Republican chairman in his area, and the second was that a Democratic state legislator had introduced a bill trying to repeal a law that side a candidate must be a member of a party for five years before running in that party's primary. When asked why he was introducing such a bill, the legislator was quoted as saying "so Chuck Whalen can switch parties."

Buchanan admitted running as a Republican in his primarily Democratic district was getting harder and harder. He said his district was mainly blud collar, and had a large black population which was "nailed down Democrat" even though Buchanan has a good record on civil rights.

He said Democrats in his district had made overtures about switching but he had not talked with O'Neil.

The third Republican could not be found. Liberal Republicans such as Reps. Silvio Conte (Mass.), Paul McCloskey (Calif.), Margaret Heckler (Mass.) and Frank Horton (N.Y.) scrambled to deny they were the one.