Can't snap modular parts together

I have made 3 simple modular units, one wall, one floor and on diagonal roof piece. I have made them in blender, they are all exactly 3x3 units and the origin point in in the centre of each individual object. I have set the grid snap in UE4 to every 10 units.
For the last hour I have tried to get the snap tool to work and I am at my wits end, I don’t understand what I am doing wrong but none of the objects will align with each other. If I somehow manage to get them to align, when I duplicate the object, it then occupies completely different increments on the snap grid.
I have also tried doing vertex snapping, however I can’t for the life of me get the origin to snap to any of the edge vertex’s, it only snaps to ones in the middle.
What am I doing wrong?