It is always great to have a few extra controllers. Although we won’t be able to use the Dual Shock 3 with the PS4, at least we will be able to use the DS4 with the PS3. Initially it was said determined that the DS4 could be used only if it was connected to the PS3 via a cable.
That definitely was going to be an annoyance considering that length of the wire. Good news now is that they have changed their plans. After the recent 4.60 update, the DS4 will work without the cable. All you have to do is sync it with the PS3 with the cable and then you can use it wirelessly.
For now all the functionalities are not available such as the six axis control, vibration or the touch pads. Other than that everything is just as it would be. You can check out the details here on Reddit.
Even if the touch controls never get activated, I guess future updates will surely allow the vibration and six axis controls to be used.