The airplane flew at an altitude of 3,000 meters.

Literal

Airplane [topic-は] 3000m [possessive-の] altitude [path-を] flew.

高度 (こうど, 'altitude') is a technical aviation term. The を marks the space through which the plane flies. Note the mixing of half-width digits and the Latin letter 'm' within Japanese text — standard in technical and scientific writing.