Can you tell me the procedure in ArcGIS 9.3 to create 3D lines which connect the 3D points using the elevation of the points. For example, if pointA has 20 elevation and pointB has 30, then the line would connect pointA to pointB vertically such that one end point of the line would have 20 elevation and the other would have 30. Or, i would like to know how to create a 3D line directly without using the points, where i can define the x,y,z of the end points.
Look into the generate commands or tools in ArcGIS. Basically you can generate a line from text as long as its in the correct format. It should be similar to 1) X Y Z, X Y Z, X Y Z end 2) X Y Z, X Y Z, X Y Z end End Where 1) 2) etc are line identifiers and end is the end of each line, the final end should end the command. This is an example drawing from memory long ago. ArcInfo has the Generate command. You should find this tool in the help docs or toolbox search.
You might want to try using the ASCII 3D To Feature Class tool. It will require that you setup a text file with the format described in the help documentation. You can also write a Python script that uses geometry objects to construct a line feature, like this example which creates a 2D line from a semi-colon delimited text file.