You can't save anything in the trial version of ReefMaster, but you can save your waypoints in either GPX or USR format in the Sonar Viewer trial. Either of these formats will import OK into the HDS3. Note that with GPX format there will be no icon information (all waypoints will import with the default Lowrance icon). With the USR (v3) format there is some (limited) conversion between the ReefMaster icon set and Lowrance where near equivalents exist (e.g. a ReefMaster fish will be converted to a Lowrance fish, bait -> bait etc.).
Waypoints can be exported from the sonar viewer using the right-click menu in the map view, and the format can be selected using the drop-down list on the file chooser window.