Solve it for y

y = 3

That's the same as

y = 0x + 3

[That's hard to recognize because the 0x is understood, not written]

Compare it to

y = mx + b

So it has a slope of m=0, and y-intercept of (0,b) = (0,3).

So it is a horizontal line with y-intercept (0,3)



Since it is horizontal it is parallel to the x-axis, and will
never intersect it.  Therefore it has only a y-intercept (0,3)
and it has no x-intercept. 

Edwin