<p>If you can’t visualize and draw in 3-D, then Calc III would be hard(er). Half the people in Calc III have taking either physics or linear algebra, and there is a lot of overlap with those classes in Calc III. Most people find double and triple integration rather easy.</p>
<p>If you struggle with u-substitution, trig substitution, integration by parts, etc. and remembering all of the countless series tests, then Calc II would be harder. You have to “see” how to solve problems in Calc II just by looking at it. Many people struggle with it because they are unsure of the proper integration method to use when they first look at a problem.</p>
<p>Just because of the quantity of information, Calc II is typically thought of as being harder.</p>