10-07E

We consider the spin-1/2 isotropic $XY$ chain in a transverse field with three-site interactions of $(XZY-YZX)$-type, which can be transformed into a system of noninteracting spinless fermions. We study dynamic properties of the spin model. In particular, we calculate the dynamic structure factors which are governed by a two-fermion excitation continuum. We demonstrate how the three-site interactions manifest themselves in the dynamic properties of the quantum spin chain. This paper was presented at the 14th Czech and Slovak Conference on Magnetism CSMAG'10 (6--9 July 2010, Ko\v{s}ice, Slovakia).