WASHINGTON (Reuters) - Three Republican U.S. senators warned Thursday they would not vote for a slimmed-down Obamacare repeal bill without guarantees that the House of Representatives will go into negotiations with the Senate on the measure. The senators - Lindsey Graham, John McCain and Ron Johnson - said they fear the House will simply take up the Senate-passed bill and approve it, instead of negotiating with the Senate to produce a more comprehensive measure. They think the slimmed-down healthcare bill, which Senate Republicans are still developing, will be an insufficient replacement for Obamacare.  The three senators are enough to block a bill from passing the Senate, assuming no Democrats will vote for an Obamacare repeal bill. 