"The experiment will employ a trio of spacecraft flying in formation in the shape of an equilateral triangle that has sides one million kilometres long, with lasers shining between the craft." The experiment in question refers to

Evolved LISA (eLISA) is the correct answer as it is a planned space-based observatory designed to detect gravitational waves using three spacecraft flying in a triangular formation separated by millions of kilometers. LISA Pathfinder is the most tempting wrong option, but it was a precursor mission involving only a single spacecraft to test the feasibility of the technology, rather than the full three-craft formation. The core concept being tested is the detection of gravitational waves through space-based laser interferometry.