3. He (be) healthier if he (not eat) so much junk food.

The sentence describes a hypothetical situation in the present, indicating a second conditional structure.

The first blank requires the past subjunctive of 'be', which is 'were' for all persons, or 'was' for 'he'. The second blank requires the past subjunctive of 'eat', which is 'ate'.

Therefore, the completed sentence is: He were healthier if he did not eat so much junk food.