What to do now?
 

Me and my wife enjoy a lot of "kinky" activities. The main one being she loves to do me with toys / strap ons. Are going to try a threesome soon, already have someone lined up, another girl, right now they are just playing with and doing each other to get used to each other before i start to join in. Have also considered another guy but one has not come along that we feel comfortable with. I am not gay by any stretch of the imagination, but I do enjoy it in the butt and she really gets off on giving anal.
Now the real question, I didnt have a hard time working up to about a 2 inch wide dildo / strap on, but now she wants to go bigger, I have tried a few things but its just to painful, how do you go bigger than that? it just seems out of the realm of possiblity, but I love to give her what she wants any ideas for stretching in a less painful way? I cannot figure out how chicks in porns can fit a fist up there, but would be up for a fist if it could fit. Also other ideas concerning kink topics to try would be helpful

Go slow. Work up to larger items over the period of a few hours.
Let pain be your guide.
You don't see any of the off-camera stuff that goes into making a porn movie.
Lube, lube, lube.

Any specific toy suggestions?

Should you be worried about the long term effects of shoving wide objects in your ass? Going past two inches is getting into some serious widths.