"I  would  stick  with  factory  loads  myself  . Nothing  like  hearing  a  click  from  a  defective  reload when a  big  tom  is  20 yards away.REloads  are  fine  for  skeet or  dove  where a  dud  dont  matter  much."
  Oh yeah, great advice from someone obviously 
NOT a reloaderand talking out of his butt! Instead, peruse the factory 3inch offerings from Remington, Winchester, Federal, etc.   
Their 20 gauge turkey (or specialty) loads 
combined with proven data from Ballisitc Products should give insight into whats possible reloading your own! I've been reloading over 40 years, and while a 20 gauge will never match a 12 gauge's performance, 
Ballistic Products offers load data and components that should interest you.  (1 1/8th ounce loads in a 3in hull,  

 )