What was the anti-submarine system that dropped two triangular patterns of three large bombs?
Short answerThe description fits Squid, an effective ahead-thrown anti-submarine mortar that followed Hedgehog and was later replaced by Limbo.
NotesThe six-bomb pattern points to Squid, which fired two three-charge salvos arranged to bracket a submarine underwater. It was an effective system in the development line after Hedgehog and before Limbo. The recognizable mortar pattern also distinguishes Squid from conventional stern-dropped depth charges.