You can use system theme to set the default line thickness as 3.
1. Select File: New: Graph to new a graph window 2. Double click the axis in the new graph window, and set the axis line thickness as 3 in Axis dialog 3. Right click on graph window and select Save Format as Theme in the context menu 4. Check Set as System Theme checkbox 5. Uncheck All checkbox and check All Styles checkbox 6. Click OK button
Now every time you create a new graph, the axis line thickness is 3.