Only if you drag and drop all the contacts from two of them into the third, and then export this third one. But it would be less work, and safer, to export them as three separate LDIF files. The main point of using LDIF to do this would be to transfer the data to another copy of Thunderbird. In that case, I'd move the whole profile over. If you use LDIF, you'll end up with five address books in a new Thunderbird. The default Personal Address Book and Collected Addresses, and the three you import. If you don't want to keep the whole profile, another approach is to copy the underlying *.mab files, and paste or import those into the new Thunderbird using the More Functions For Address Book add-on. Then you'll have just the same three address books. If you are not going to be using Thunderbird on the new computer, then LDIF or mab files are unlikely to be of any use to you. In this case, you should create three CSV files.