A new update has released for the Windows PC version of the PlayStation Now app, which allows players to stream PlayStation Now titles on their PCs. This particular update now allows for the use of DualShock 3 controllers. Previously, only the PlayStation 4 controllers could be used. Since the DualShock 3 controllers can be used now, the PS3 games with SIXAXIS motion support can now be fully utilized with the DS3 controller as one would do on the PS3 originally. 
Please note that the PlayStation Now support page still claims that the Windows PC app does not support the DualShock 3 controller. However, the PlayStation Help site says that DualShock 3 support for the PC app is now included.

The PS Now app for Windows PC includes full support for the DualShock 4 and DualShock 3 controllers. Connect the controller via USB mini cable via a USB port on your PC to use PlayStation Now.
Wireless support, as you can see, is not yet available for the PC app. Every other feature of the DualShock 3 controller is fully supported, however. Sony did additionally note that third-party controllers may not work as well with the app.

Some third party PC compatible controllers that use XInput will work with the PS Now on PC service, but we do not guarantee that they will have full compatibility with the PS Now on PC service. For example, some third party controllers will not support motion control, or other features.

[Sources: PlayStation Help, Pure PlayStation]
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