My samsung Galaxy s3 has its screen cracked. Is it worth putting a replacement screen on myself?

No need to worry, If you want to repair by own, so there are following steps:
Remove the back cover.
Remove the battery, SIM card and microSD card.
Unscrew the 10 screws holding in the plastic housing.
Pry off the plastic housing using the plastic wedge tool.
Disconnect the connector for the headphone jack at the top of the circuit board.
Remove the plastic housing under that connector.
Unscrew the black screw at the bottom holding in the circuit board
Remove all the connectors attached to the circuit board and the bottom end of the white antenna
Remove the circuit board.
Remove the screw at the top.
Remove all the housing at the top and the antenna
Remove the vibration motor at the bottom.
Remove the front facing camera at the top.
Pop out the Volume Rocker and the Power button.
Put all of the removed hardware from the old device into the new device, replace all the screws and housings, and replace the battery and back cover.

Hi Cement,

Sorry to hear about your Galaxy S3! Depending on the damage (if you can see the the video underneath and the touch feature works), some repair shops can fix your device for much less than the cost of a new phone. There is a company called Phone Doctors (www.PhoneDoctors.com) that offers a lens-only replacement for $150 via their mail-in repair service. These guys are AWESOME and fixed my friend's phone in no time at all. If for some reason your video is not displaying or your touch is not working, the worst-case scenario would be about $240 for a new screen (which is the LCD and Digitizer + Lens as a combined part).

I do not recommend trying the "glass only" fix as you run the risk of really messing stuff up unless you have the correct equipment, which Phone Doctors has. However, this repair can most likely extend the life of your device! :)