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Roku allows third party developers to write apps for its platforms.  There are several apps that you can download to use your Apple iPhone 5s as a remote control that connects to your Roku 3 over WI-FI.

You can go to the iTunes Store to download one of these apps like RMote Roku Remote.

You need to make sure both the iPhone, in this case an iPhone 5s is connected to the same WI-FI network as the Roku 3.

Once they are connected to the same network, you should be able to use your remote control App to find the Roku and connect to it.

After connecting, your iPhone now becomes an alternative remote control for your Roku.