(as it says above the door of the old V3 freeware section - my italics)SyncBack freeware has been retired and replaced with SyncBackFree. These old posts are being kept for reference only. SyncBack V3 freeware is no longer supported nor available for download.
For what you say you want, simply click New, give it a Name, select Mirror, then assuming your devices/resources each side are both standard disks or network paths, simply click Done (instead of Next).
The profile Modify window will open for you to enter (or Browse to) your Source & Destination