SANTA ANA โ Three 18- year-old county youths ac cused of stabbing to death two brothers last July as they were riding a bicycle near El Salvador Park here pleaded guilty to lesser of fenses Friday in Superior Court. The three, charged with murder, agreed to plead to lesser offenses with the understanding they would be sentenced to no more than six months 1 county jail and three to five years probation Judge James K. Turner ordered a probation report to be prepared and Scheduled sentencing for Nov. 1 A condition of that sen tencing, said prosecutor Ted Millard, would be con finement to state prison for the full term prescribed by law for the lesser offenses if they violate probation Entering pleas were Randy Ponce. 910 ยง Main St, who pleaded guil ty to voluntary manslaugh ter and admitted he was armed with a deadly weapon at the time of the deaths . Steven Acala, 1133 W. Chestnut St., and Alex Seg ura, 1325 W. Ninth St., who each pleaded guilty to as sault with a deadly weapon. Testimony in the premi nary hearing indicated that only Ponce was armed at the time of the homicides, said Millard, and there were circumstances which had a bearing on the ac ceptance of the pleas. Millard said witnesses to the July 24 incident in the 500 block of north Western Avenue testified the two victims. Hector Dolores, 17, and his brother, Walter, 15, had a small caliber pistol while bicycling in the park and fired four or five shots at persons in the park as they rode past Following the shooting, the witnesses said, several persons including the de fendants, gave chase to the thievelists and caught them No pistol was found at the scene, Millard said, but olce discovered small caliber pistol ammunition in the pocket of one victim and a bullet wound in his leg