  The Landauer-B\"uttiker formalism establishes an equivalence between the
electrical conduction through a device, e.g., a quantum dot, and the
transmission. Guided by this analogy we perform transmission measurements
through three-port microwave graphs with orthogonal, unitary, and symplectic
symmetry thus mimicking three-terminal voltage drop devices. One of the ports
is placed as input and a second one as output, while a third port is used as a
probe. Analytical predictions show good agreement with the measurements in the
presence of orthogonal and unitary symmetries, provided that the absorption and
the influence of the coupling port are taken into account. The symplectic
symmetry is realized in specifically designed graphs mimicking spin 1/2
systems. Again a good agreement between experiment and theory is found. For the
symplectic case the results are marginally sensitive to absorption and coupling
strength of the port, in contrast to the orthogonal and unitary case.
