Individual Axis: This is mainly used when only one measure is used in single line graph. One axis represents one measure information. Example: Sales by Year. Sales can be on one single axis with the range of values from source data.
Dual Axis: This is mainly used when two mesaures are used in dual lines graphs or charts. One axis represents one measure and other axis represents second mesaure. Both axis will be parallel to each other with different range of values from the source data. When we select dual lines template from Tableau Show me list, by default it implements dual lines graphs or charts using dual axis concept. Example: Sales and Profits by Year. In this case the sales and profits are represented in two different axises.
Blended Axis: This is mainly used when more than two mesaures are used in multi lines graphs or charts. Also, when there is a need to show two measures on the same axis, then Blended Axis is the option to explore. We need to drag drop the scond, third and other mesaures in to the first axis of the measure to blend the multiple measures in to one. In this case the single blended axis with multiple measures contains the range of values from the source to satisfy all the measures those are blended in to one axis. The name of the axis changes in to 'Value' which generic in nature. Example: Sales, Profits and Discounts by Year.