see this page:https://www.roku.com/accessories
You want remote 4th on the list, the Enhanced Voice Remote, it doesn't list roku 3, but it WILL work. If you don't need headphone jack on the remote, and want to save $10.00 go for the second remote, the Roku Voice Remote. its thinner then then your roku remote.
The fifth remote is listed as Roku 3, but its just like one you already have no volume, only remotes listed with Tv Power & Volume have that feature.