Transfer the online character from the PC to PS3

Nope. The platform you bought it on is the one you have it on. There is no way to transfer it to a different platform. All you can do is buy it for any other platform you wish to play it on.

According to the information from Rockstar Support found here, You can transfer your progress to next generation consoles(Xbox one, PS4) and to a PC with the help of your Rockstar social club account. They have not mentioned about character transfer from PS3 to Xbox 360 in the provided link.

You can give it a try if you want with the procedure mentioned in their support link which I have given but, you will have only one chance and there are lot of limitations. Proceed at your own risk.

You can also post a question in Rockstar support for a definitive answer for your question.