Hey kingsolomonsgrave.

You will want to create a definition for the function at x < 3 and one at x = 3 and possibly at x > 3 if you want a point discontinuity as opposed to just a normal one.

You can use what is called the Heaviside function to do this:

Heaviside step function - Wikipedia, the free encyclopedia

It basically does what a piece-wise function does by transferring this information into H(x-a) where H is the Heaviside function.