I had an old computer going back some years ago and had to break a lake map into 3 segments. Now with a new computer, I would like to try to import a new complete shore line and add all my soundings from my 3 segments maps to make 1 complete map. The lake is 12 kilometers in size. Can I do this and if so how?

It shouldn't be too hard to do this, but if you have three separate shorelines now then you will probably need to create a new shoreline that covers the entire map area. Once you have a new shoreline, you can just import all of the data for the 3 old maps and build as normal. If you need to export data from your old ReefMaster workspace, then you can multi-select all of the required tracks and export in a format of your choice, CSV probably being the easiest.