Captain Surgeon Graeme Nicholson told an inquest that official guidelines he authored "should have been followed".
L/Cpl Craig Roberts, 24, of Conwy county, L/Cpl Edward Maher and Cpl James Dunsby died after attempting a 16-mile march in the Brecon Beacons on one of the hottest days of 2013.
He said assessors should have been aware of environmental conditions.
Dr Nicholson wrote the Army's guidance document on climatic illness and injury.
He told the inquest in Solihull, West Midlands, that this was "the standing guidance" and should have been followed by soldier 1A and 1B, who were in charge that day.
This document was primarily used outside the UK in hot conditions, he said.
Coroner Louise Hunt asked how he felt about many witnesses not having any knowledge of the guidance.
"Frustration is the honest statement," he replied.
Keith Morton QC, representing Cpl Dunsby's father, asked: "It's completely unacceptable isn't it that those on the ground were ignorant to it?"
Dr Nicholson said: "That again I would refer back to their chain of commandâ€¦ I do not feel from a personal point of view that it was appropriate."
The inquest continues.
A military doctor was "frustrated" by a lack of knowledge of heat illness policy on a fatal SAS selection march.