Welcome to the forum.
For CS 3, I would go to the ProDesign Tools Web site, and download the trial for Ps CS 3. Be VERY SURE to read the full instructions carefully, and follow them faithfully.
Then, you should be able to Activate the trial with your Ps CS 3 S/N.
PS - I am going from memory, that the Ps CS 3 trial can be activated to "full-paid" with the S/N. I know that at one time, the Activation of a trial was not possible, but THINK that Ps CS 3 allowed that - if not, others will correct my memory.
If you do not have s/n and you registered it, can find on Adobe/my stuff/programs