He could have used sedatives on all three of them.
He could have waited for two of them to commit suicide and only kill one of them afterwards without a struggle
He could have simply been the one to switch on the electrical rig on the man that was planning to commit suicide by means of electricity before he was ready counting as murder.
The definition of murder could include assisted suicide, which means a fourth person could have switched on the electricity or shot one of them while they were perfectly willing and have it considered as murder.