The sentences with incorrect syntax are:
- "The boy eight an apple."
- "Boy the apple an ate."
- "The boy 8 an apple."
Among these, "The boy eight an apple." and "The boy 8 an apple." are incorrect due to using "eight" and "8" instead of "ate." "Boy the apple an ate." is also incorrect due to a disorder in word order and grammar.
Overall, all three sentences have incorrect syntax.