Step 1: Common confusion check:
The exponent 2 on \(\left(\dfrac{d^3y}{dx^3}\right)^2\) is the *degree* contribution, not the order — order never depends on powers/exponents.
Step 2: Re-reading only derivative levels:
Ignoring all powers, the derivative levels present are 3rd, 2nd and 1st order; the largest is 3.
Final Answer:
\[ \boxed{3} \]