Well it all depends...there's no rule on which picture you blend into which.
Here I made the middle picture the bottom layer and blended the other two pictures into it:
But here I made the middle picture the top layer and blended it into the other two pictures:
And here I the Eowyn picture is the bottom layer, the middle picture is the 2nd layer (which I blended into the Eowyn picture) and the Eomer picture is the top layer which I blended into the middle picture.
You can do it a whole bunch of different ways. I would suggest making your top layer the picture that you want to be the 'center of attention' so that it's like the foreground. But try it out and see what happens. Some picture just blend better in a certain order, so if your trying to blend two pictures and it's just not working then delete them and try to blend them in reverse order. Honestly I don't put much thought into what the top and bottom layers are. I just put them on there and blend. Then if it doesn't work or I don't quite like how it works I switch the order. I hope that helps
Good luck!