Trishul missile system is not a failure. Technically it has met the performance objectives as per original Qualitative Requirements (QRs) though delayed. The main reason for this delay are technical problems in achieving perfect 3-beam guidance and obtaining millimetric wave components from abroad. Trishul missile project is required even after the successful test of firing of Agni-3 Missile ( you can just imagine the question from the MP!).
Trishul missile is a low-level, quick-reaction, short range, surface-to-air missile against air targets whereas, Agni-III is a long range surface-to-surface missile against fixed strategic targets. As the realization of Trishul missile system got delayed, users (Army, Air Force, and Navy) resorted to acquisition from abroad as the QRs and operational requirements got modified over time. Dialogue is still on with Air Force to accept the system developed and produced for deploying in certain locations and for training/fire control/air defence practices.
(Training! Now, this is new… a bit like General JJ Singh saying that the Army will sincerely find a place where it is suitable for the Arjun MBT to be deployed ).