Are you planning on racing or just keeping for the love of them? Pigeons like any creature on earth can get a disease but most are very easily treated and prevented with the proper cleanliness, vaccines, and diet. 3 pair will produce you 18 young/Year comfortably on them. A 6x6 loft would be enough for the 3 pair themselves without many young. First thing you need to ask yourself is how many you plan on having, what you are going to do with them, and then read alot of the post on here to learn as much as possible prior to getting any. Remember this is not a cheap hobby and the birds depend on your finances to keep them healthy and fed.
Ken
Ken