The square root of 3 is the positive real number that, when multiplied by itself, gives the number 3.It is more precisely called the principal square root of 3, to distinguish it from the negative number with the same property.It is denoted by √ 3.. The square root of 3 is an irrational number.It is also known as Theodorus' constant, named after Theodorus of Cyrene, who proved its irrationality.