  We show that c-planarity is solvable in quadratic time for flat clustered
graphs with three clusters if the combinatorial embedding of the underlying
graph is fixed. In simpler graph-theoretical terms our result can be viewed as
follows. Given a graph $G$ with the vertex set partitioned into three parts
embedded on a 2-sphere, our algorithm decides if we can augment $G$ by adding
edges without creating an edge-crossing so that in the resulting spherical
graph the vertices of each part induce a connected sub-graph. We proceed by a
reduction to the problem of testing the existence of a perfect matching in
planar bipartite graphs. We formulate our result in a slightly more general
setting of cyclic clustered graphs, i.e., the simple graph obtained by
contracting each cluster, where we disregard loops and multi-edges, is a cycle.
