This process used to be an arcane Windows trick to link folders between two physical drives. These days however, Steam has the necessary functionality built in.
These steps will move a game from the default location (usually C: drive) across to a second drive you've installed (shown as R:\ drive in the example below). Before following the instructions, ensure your new drive is installed and accessible to Windows as a new drive letter, and plenty of disk space available for the game(s) you're going to move across.
1. Open Steam
2. From the main Steam menu, select Settings
3. In the Settings panel, select Downloads
4. Press the STEAM LIBRARY FOLDERS button
5. Press the ADD LIBRARY FOLDER button
6. Drop down the listbox at the top of the next dialog
7. Select the drive letter assigned to your new SSD
8. Press the NEW FOLDER... button
9. Type the name of the new folder that will store your Steam games in, on the SSD
10. Click OK
You should then see the new folder as an entry in the Steam Library Folders list.
The next step will move Arma 3 files from your C: drive, to your new SSD...
1. Open Steam and select the LIBRARY tab
2. Right-click Arma 3
3. Select Properties
4. Select the LOCAL FILES tab
5. Press the MOVE INSTALL FOLDER... button
6. Ensure the correct destination folder is selected, and press the MOVE FOLDER button.