Step 1: Link $pK_a$ to acid strength.
A lower $pK_a$ means a stronger acid. So the question is really asking which structure is the strongest acid.
Step 2: Know what makes an acid stronger.
An acid is stronger when the negative charge left behind after losing $H^+$ is well spread out and stabilised. Electron withdrawing groups pull electron density away and stabilise this anion.
Step 3: Spot the electron withdrawing groups.
Among the structures, look for strong electron withdrawing groups like the nitro group ($-NO_2$). Such groups greatly stabilise the conjugate base.
Step 4: Compare the structures.
The structure carrying the strong electron withdrawing nitro group at the para position stabilises its anion the most, making it the strongest acid.
Step 5: Pick the strongest acid.
The para nitro substituted acid (Structure 3) releases its proton most easily, so it has the lowest $pK_a$.
Step 6: State the answer.
So Structure 3 has the lowest $pK_a$.
\[ \boxed{\text{Structure 3}} \]