I'm guessing you are talking about the models in Mouser Electronics - Electronic Components Distributor, I was able to find the EDI2ARDUIN.AL.K but not the EDI3ARDUIN.AL.K. Could you post a link to this reference?
The available expansion boards for the Edison module are: The Arduino expansion board and the mini breakout board.
If you are talking about the models in Mouser's website, you could check for the differences between the models. Most of the times you will find that the models have the same price and same specifications/features [For example: EDI2.LPON.AL.S and EDI2.LPON.AL.MP] the only difference is in the the certifications of the product, this is in order to be sold in more countries. If you have more questions about what they are offering in their website (If you are talking about Mouser) I suggest you to contact them and ask for the differences between the models and to help you to choose which option will work for you according to the country where you are placed.
I hope this helps, if you are not talking about the models in Mouser, please provide us the links where these products are.