As @oliverschmid mentioned, it depends on your longterm plans.
If I had 3 Legendaries of the same type, I would combine 2 of them but I would keep the 3rd card untouched for later selling. I assume that there will be a option to "delegate" or "rent" cards to other players, so I could play with the one card for myself and delegate the other card to a member of the own guild.