Gandalf and Saruman are names inspired by Old English. Gandalf means "wand elf" and Saruman means "skillful man". Tolkien explained that these names were translations of the true Westron names, which were not given. He often used Old English to translate archaic Westron names, just as he used modern English to translate Westron.
Sauron on the other hand is elvish, meaning "abhorred".