I stood on the beach. The waves rushed to shore. The seagulls flew overhead. (Combine into a compound-complex sentence.)

While I stood on the beach, the waves rushed to shore, and the seagulls flew overhead.

Explanation:

compound-complex = (at least one) dependent clause + two independent clauses connected together

One of the three original simple sentences (independent clauses) was converted into a dependent clause by starting it with an adverb (while).

As I stood on the beach, the waves rushed to shore, and the seagulls flew overhead.