The Washington Redskins, who last week picked 12 players in the National Football League draft, have signed 15 more as free agents, General Manager Bobby Beathard announced yesterday. One is rather unusual: Ron Peterson, a 6-foot-10, 300-pound offensive tackle from the University of Nebraska at Omaha who was all-America both in football and trapshooting.

Peterson, who also was his team's long-snapper, was a first-team Kodak all-America (college division) in football and a member of Sports Afield's all-America trap-shooting team in 1981-82.

"I haven't been trapshooting in a while," said Peterson, who played at 290 pounds in college but has added 10 pounds by lifting weights. "For now, I'm concentrating on football. I think it's my better sport."

Two members of Brigham Young's national championship team were signed as free agents: linebacker Marv Allen (6-3, 230), an academic all-America, and tight end David Mills (6-2, 225), who was a member of the Western Athletic Conference's first team and was honorable mention all-America. Mills, who led BYU with 60 receptions for 1,023 yards last season, had 11 catches for 103 yards in the Holiday Bowl.

The Redskins signed three wide receivers: Southern Illinois' Tony Anderson (5-10, 175), Texas-Arlington's Andre Gary (6 feet, 200), and Rutgers' Boris Pendergrass (6 feet, 175), who also ran hurdles in college.

The other free agents were centers Dan Eernissee (6-3, 247) of Washington, Bill Legg (6-3, 265) of West Virginia, and Mike Wooten (6-3, 253) of Virginia Military Institute; safeties Mike Kenealy (6 feet, 187) of Central Michigan and Kurt Kafentizis (6-2, 190) of Hawaii; defensive tackle Don Coleman (6-4, 265) of Murray State; guard Tony Kepano (6-1, 258) of Georgia Tech out of Gar-Field High School in Woodbridge; running back Mike Newton (5-11, 227) of Southern Connecticut, and linebacker Alvis Satele (6-1, 223) of Hawaii.

They will join the team at the week-long minicamp, which will begin Monday at Redskin Park.