Assuming Gandalf was the wisest of the maiar, why was Saruman known as the wise, rather than it being Gandalf?
Also, what are the real names (maiar names) of all these three (Sauron, Saruman and Gandalf)?
It's possible to be wise without being wisest. At 6 foot one I'm tall, but other people are taller.
In Tolkien's works Elendil was known as "the tall", but Thingol was stated to be "tallest of all the Children of Iluvatar". It's not really a problem.
For "real" names, Gandalf's is given in multiple places in Lord of the Rings, for example in Faramir's discussion of him:
Olorin I was in my youth in the West that is forgotten...
This is confirmed by the Valaquenta in the published Silmarillion, where it is said of Olorin:
Wisest of the Maiar was Olorin. He too dwelt in Lorien, but his ways took him often to the house of Nienna, and of her he learned pity and patience.
Of Melian much is told in the Quenta Silmarillion. But of Olorin that tale does not speak; for though he loved the Elves, he walked among them unseen, or in form as one of them, and they did not know whence came the fair visions or the promptings of wisdom that he put into their hearts. In later days he was the friend of all the Children of Iluvatar, and took pity on their sorrows; and those who listened to him awoke from despair and put away the imaginations of darkness.
Even that isn't a true real name, however, as the names of the Valar and Maiar are all of elvish form, but the original Valarin forms of most are unrecorded and we must be content with Olorin.
Saruman's name is given in the Istari material in Unfinished Tales as Curumo, and Sauron's in a semi-obscure periodical (Parma Eldalamberon): Mairon.