Is x = 3 a function?

no, it's not

Step-by-step explanation:

x=-3 is a vertical line that is parallel to the y-axis

this is a relation but it is not a function

to be a function, for every x value there can be only one y value

on the vertical line x=-3, when x=-3, y can be 0 or 1 or 2 or - 5 or any other number

y=x^2 is a function

if x=2, y=4

if x=-2, y=4 also

you can have different elements of the domain go to the same element in the range

you cannot have the same element in the domain go to different elements in the range

this is what you have with x=-3

-3 goes to any number in the range

in a function you can have "many to one" but you cannot have "one to many"