Vintage 1970's era stadium seat cushions in general sell for about $10.-$25. The Oakland A's had emerged as one of Baseball premiere dynasties in the 1970's winning the World Series in 1972,1973, and 1974. The three seat cushions you have commemorating the three world championship years should hold higher value.
The stadium seat cushions might be rare but so would a collector of seat cushions. The bulk of the value will rely on the fact that they are World Championship souvenirs. I left a link below for Miscellaneous Baseball Collectibles. Check it out for similar items.
World Heavyweight Championship Joe Frazier vs- Ron Stander - 1972 TV was released on: USA: 25 May 1972
1972
1972 American League Championship Series - 1972 was released on: USA: 7 October 1972
1972 NFC Championship Game - 1972 - TV was released on: USA: 31 December 1972
1972 AFC Championship Game - 1972 TV was released on: USA: 31 December 1972
1972 NFC Championship Game - 1972 TV was released on: USA: 31 December 1972
1972 National League Championship Series - 1972 was released on: USA: 7 October 1972
1972 Formula 1 champion: Emerson Fittipaldi (Brazil)
Richard Petty won the Nascar Winston Cup Series championship in 1972.
1972 American League Championship Series - 1972 Game 1 was released on: USA: 7 October 1972
1972 American League Championship Series - 1972 Game 2 was released on: USA: 8 October 1972
1972 American League Championship Series - 1972 Game 3 was released on: USA: 10 October 1972