No, Adobe won’t sell obsolete versions of Photoshop.
You could install the 30-day-trial, but to (edited) use it permanently I think you would need to buy a license from another user.
The necessary transfer-of-license would involve some »paperwork« to get regisered at Adobe – inconvenient, I guess, but possible.
If I would like to buy it can I get it from Adobe site?
Adobe only sells the current version which is CS6.
Any reason why you want CS3 rather than CS6?
And just to confirm once more what the otehrs said: No, at this point Adobe does not sell CS3 nor does any officially authorized reseller. Your only option is to buy a used license and do a transfer of the product license.
And just to confirm once more what the otehrs said: No, at this point Adobe does not sell CS3 nor does any officially authorized reseller.
This is an important point. There are vendors who still sell PS CS3 extended (currently on Amazon.com marketplace there are some available for between $600 and $850), but they are probably NOT authorized resellers. A high price is no guarantee that the software is legitimate (e.g. not pirated or not licensed to another user).