Mobil Oil Corp., whose chances of buying Conoco Inc. were dealth a severe blow last week by the Justice Department's antitrust lawyers, today added $430 million to its $8.2 billion offer for Conoco to stay in the three-way battle to acquire the nation's ninth-largest oil concern.

Mobil boosted its cash offer for half of Conoco's more than 86 million shares from $105 to $115. It will still offer Mobil securities worth $85 a share for the rest of the stock.

Wall Street analysts said the three-way battle among Mobil, E. I. du Pont de Nemours & Co. and Seagram Co. still looks as if it will go Du Pont's way. Although it is by far the richest, the Mobil offer may not be high enough to overcome the risks that the antitrust department will kill the deal, analysts said. Mobil said today it will buy shares and take the risk itself that the government will kill the deal unless it is under a court order preventing it from making payment to Conoco shareholders.

Du Pont has offered a package of cash and stock worth $7.3 billion for all of Conoco and Seagram wants to buy 51 percent for $4.08 billion. Du Pont said it would respond to the new Mobil offer Tuesday.

Du Pont has a tentative clearance from the Justice Department and plans to start paying $95 a share for 38.7 million shares on Wednesday, the day after Conoco stockholders rights to withdraw any shares they have sent to Du Pont expire. Stockholders who sent their shares to Seagram had their withdrawal rights expire Friday. On Saturday Seagram began issuing checks for $92-a-share for 15.5 million Conoco shares it had irrevocably in its control.

Meanwhile, Du Pont apparently cleared the last potential federal roadblock to acquiring Conoco by agreeing to buy Monsanto Co.'s interest in a Du Pont-Monsanto venture. The Justice Department said late yesterday that it will file a decree that it has no antitrust objections to a Du Pont-Conoco merger if Du Pont completes its deal with Monsanto within 10 days of acquiring Conoco.

Today's Mobil offer was sufficiently lucrative to drive Conoco stock that is still trading on the open market to $93.50 today, $1.50 more than Seagram is paying. That kept most Conoco owners who had not sent their stock to Du Pont or Seagram on the sidelines.

Seagram today made no changes in its offer and is paying $92 on a first-come, first-served basis to anyone tendering a share of Conoco.