First, see this post by @foosel:
Second, you do not need a "dedicated" Windows machine. Repetier-Host functionality is replaced by OctoPrint running on your Raspberry Pi 3 and installing a slicer (Cura) on your regular desktop machine is the normal configuration. In Cura, you will slice your .stl file, Save to File, and use the "Upload" button at the bottom left of the main OctoPrint screen (in your browser) to put the file on the RPi and from there you can print it.
The Raspberry Pi 3 is "dedicated" to running OctoPrint and controlling your printer. Your desktop computer is used to generate .gcode files for OctoPrint. 3D printing is a time consuming process and it is not uncommon for print times to be measured in (multiple) hours.
As you progress in your 3D printing adventure, you will discover that you don't want to be stuck with printing everyone else's designs and you will want to design something of your own. Your desktop computer is the appropriate place for this activity. I started with Tinkercad.