With about 70 percent of the delegates chosen, it now seems certain that the Republican state convention in Richmond June 2-3 will be a wide open, three-way contest for the U.S. Senate nomination.

The managers and supporters of all three leading candidates give a delegate lead at this point to Richard D. Obenshain, a Richmond lawyer and former national GOP cochairman.

However, it appears likely that he will go to the convention about 350 votes short of the 1,541 needed for a first-ballot victory. This means his principal opponents, former governor Linwood Holton and former Navy secretary John Warner will each go to the convention as strong contenders.

Only state Sen. Nathan H. Miller, who is given well under 100 delegates by all counts so far, is considered out of the running.

Of the more than 2,000 delegates chosen so far, Obenshain has more than 800 by the count of his campaign director, Judy Peachee, and more than 700 by the count of Warner's manager, Bill Russo. Holton's campaign director, Cathleen Lawrence, said it is Holton's policy to avoid vote estimates.

The Republican delegates are being chosen at city and county "mass meetings" that began in mid-February and will end in mid-May.

The big disagreement among campaign figures now is over who is in second place. Russo claims about 650 votes for Warner, making him a close second. Peachee said she puts Holton in second, leading Warner by 407 to 336 votes as of last Wednesday night. Lawrence said Holton leads Warner, but again declined to say by how much.

The number of uncommitted delegates is estimated at between 175 and 275 by the campaign officials.

Firm counts of Republican delegates are hard to obtain because all but about 14 percent of those chosen so far have been elected without having to make a binding commitment to vote for any one candidate in the Senate race.

Only two large delegations, a total of 269 votes from the Richmond suburbs of Chesterfield and Henrico counties, have been chosen under instructions to vote for one candidate, Obenshain.

A slate of 98 delegates publicly commited to cast all of its votes for Holton was selected in Norfolk. In Virginia Beach, a slate of 111 was chosen, some committed to Holton and some to Warner. However, these slated delegates are free under convention rules to change their mind.

Obenshain's success in winning instructed delegations in Chesterfield and Henrico gave him an overwhelming majority of the 418 delegates in his home Third Congressional District. He won about 90 votes in Richmond giving him a Third District total of about 369, or about 45 percent of all the votes he has collected so far.

If Obenshain continues to win the same share of votes outside his home area that he has won up to now, he should end up with about 1,200 before the convention begins.

Russo, however, claims that Warner is doing better than Obenshain in the small cities and rural counties that have the bulk of the remaining delegates. The Holton campaign is hoping for a decisive victory in Roanoke and Roanoke County, which was Holton's home political base when he was elected governor in 1969. He now lives in McLean.

Republican convention rules permit all candidates to remain in the race until one wins a majority. This means the process for selecting a nominee to run for the seat being vacated by the retirement of Republican Sen. William L. Scott could go for several ballots before a decisive movement of delegates from one candidate to another begins.

The chances that one of the three leaders will decide early to throw his support to one of the other two are thought to be slim by Republican officials who know them all. "These are three proud men," one said yesterday. "None of them is going to pull out until he is pushed out."