  Inert, spherical heterogeneities can pin three-dimensional scroll waves in
the excitable Belousov-Zhabotinsky reaction. Three pinning sites cause
initially circular rotation backbones to approach equilateral triangles. The
resulting stationary shapes show convex deviations that increase with
decreasing anchor radii. This dependence is interpreted as a transition between
filament termination at large surfaces and true, local pinning of a continuous
curve. The shapes of the filament segments are described by a hyperbolic cosine
function which is predicted by kinematic theory that considers filament tension
and rigidity. The latter value is measured as (1.0 $\pm$ 0.7)$\times$$10^{-6}$
cm$^4$/s.
