On the reference tetrahedron K, we define three projection-based interpolation operators on H 2 ( K), H 1 ( K, curl), and H 1 ( K, div). These operators are projections onto spaces of polynomials, they have the commuting diagram property and feature the optimal convergence rate as the polynomial degree increases in H 1−s ( K), H −s ( K, curl), H −s ( K, div) for 0 ≤ s ≤ 1. * {