The slope intercept form of the line is:
y=mx+b, where m=slope and b=y-intercept
In this case y is equal to a constant, which is also the y-intercept and is of the form:
y=k
Note that there is no mx part to that line, so for all values of x, y=3. And the only value for m that would make mx=0 is for m=0
Since m=0, there is a slope of zero. This is just a horizontal line.