I want to know if I can download Adobe photoshop CS3?

Of course you can. But if you're friend wants to use it I guess that friend will take courses? If so then he will get PS at the center. If it is only for making collage and so on then I suggest torrent with positive comments. Be sure to disable autorun first then install it with a vaccine program.

Yes, you can but you would need a product key to activate it afterwards. Like with most software, you will get a free trial of about a month when you download it and skip the entering of the licence key. I believe you get full use of all the features in the Photoshop program but after the trial period ends, you will no longer be able to use it unless you type in a valid serial number (registration key).

There are cracked versions of the software but you might end up getting viruses/trojans while looking for them.