<p>Why would you take Calc III and DiffEq in the same semester? Seems like having Calc III under your belt would be helpful for DiffEq.</p>
<p>As for the relative ease of these courses, I really think it depends on the sort of mind one has. I thought DiffEq (taken at your U, boneh3ad 
was the easiest math class I ever took. My PhD astronomy friend struggled with it a bit. It just depends.</p>
<p>If a 4.0 is your goal, stick with your plan.</p>