I have hunted all three and I have hunted Rio's, Merriam's and Easterns in multiple states. As far as which is tougher to kill. To me it is all about pressure, birds that get more pressure are tougher to kill.

Specific differences to me are Merriam's and Rio's are more likely to be in larger groups and multiple toms. This does help to increase the competition factor and may be a reason the are more responsive to a call. However, I still believe it is largely a factor of they are usually hunted less.

Osceolas are tough birds to kill, but not as tough as they are often portrayed. They do gobble less, but will gobble and work a call. I have also killed a few Osceolas that were racing to my call. I believe that they are harder to hear. The heavy air and thick foilage eats up gobbles, so they may gobble as much as Easterns we just here them less.

Easterns are generally the toughest bird, but they are also the birds that I hunt that have and continue to receive the most pressure. They gobble a lot, but are more reluctant to come to a call.

That is the biggest difference to me. Pressurred birds don't walk as far to see a hen. I also believe out west the birds are at a slight disadvantage because you can often see them in the distance, but you have enough terrain to make a move which would put you in a better position to kill them.