All three clubs have slid out of the top flight in recent years and were last season relegated from the Championship.
With Oldham and Leeds - both former Premier League outfits - also in League One again this coming season, the race for promotion looks tougher than ever.
Gregan, 34, said: "I was playing in the Premier League when some of those clubs were right up there and they won't relish competing at this level, believe me.
"It is not an easy league to get out of, just ask Leeds United. It will be tough, but we are all looking forward to it."