In this paper, we give the notion of a CLWX 2-algebroid and show that a QP-structure of degree 3 gives rise to a CLWX 2-algebroid. This is the higher analogue of the result that a QP-structure of degree 2 gives rise to a Courant algebroid. A CLWX 2-algebroid can also be viewed as a categorified Courant algebroid. We show that one can obtain a Lie 3-algebra from a CLWX 2-algebroid. Furthermore, CLWX 2-algebroids are constructed from split Lie 2-algebroids and split Lie 2-bialgebroids.
Comment: 27 pages, notations are changed, we use the terminology "CLWX 2-algebroid" instead of "LWX 2-algebroid", the construction of a transformation CLWX 2-algebroid is given, to appear in J. Symplectic Geometry