√x + 3 is a pair of real numbers for all values of 'x' that are zero or greater.
√(x + 3) is a pair of real numbers for all values of 'x' that are -3 or greater.
For other values of 'x', there is a whole system of so-called imaginary numbers
that you will learn about in future math classes. With those, you'll be able to
handle square roots of things that may sometimes be negative.