As relations with local forces continue to deteriorate the deaths mean that a quarter of all British fatalities this year have been caused by Afghans soldiers with seven murdered at the hands of allies.
The soldiers, two from the Welsh Guards and one from the Royal Corps of Signals, are understood to have become involved in an argument with one of the Afghan policemen.
The officer opened fire, probably with an AK47 machine gun, and hit three of the soldiers. It is unclear if they were wearing body armour at check point Kamparack Pul in Nahr-e-Saraj in Helmand. A fourth British soldier was wounded but is thought to be stable.
The Afghan policeman was then shot and wounded. He is now in custody.
The deaths bring the total number of British fatalities in Afghanistan to 422.
In a statement the MoD said: “The soldiers were serving in an Afghan Police Advisory Team and had been to the check point to conduct a shura (meeting). On leaving, they were engaged by small arms fire by a man wearing an Afghan Police uniform. During this exchange of fire the three soldiers were wounded and despite receiving first aid at the scene, they died of their injuries.”
The killer was a member of the Afghan Civil Order Police, whose members have a reputation for having better training and greater discipline than the notorious national police.
A police source in Helmand said shooting had erupted after an unknown argument.
He said: "There was an argument between the foreigners and the Afghans. There was a lot of shouting and then suddenly there was shooting."
Ghulam Sakhi Ghafoori, head of the civil order police in Helmand, said his men had been partnered with British forces to secure a local highway.
He said: "We don't know what happened yet. The British and my police have been working together. I am on my way to the scene to investigate."
While Afghan police are supposed to heavily vetted a number of what is called “green on blue” incidents have happened in the last year.
The incident is the deadliest since 2010 when an officer two Gurkhas were killed by an Afghan soldiers. That incident came eight month after five soldiers from the Grenadier Guard battlegroup were murdered by a policeman.
Commanders are now calling for a change in policy by adopting an American approach to security when working alongside Afghans.
The shootings will also be a severe set back in the training and mentoring programme of Afghan security forces. British troops are now “embedded” with Afghans across the local army and police, some in small numbers.
But following an incident in Kabul earlier this year the Americans have adopted a policy that at all times during meetings or operations at least one soldier is stood back with his weapon “cocked and locked”. However, this has raised issues of trust with local troops.
“The concern is going to be where is the whole mentoring and training programme going to go with ‘green on blues’ happening with this regularity?” said one officer.
“Look at what happened when the French had green on blue with four dead, they announced within hours that they were leaving Afghanistan.”
Major Ian Lawrence, the British military spokesman said: "Their loss will be felt deeply across Task Force Helmand. However, this will be nothing compared to the grief experienced by the soldiers’ families. Our thoughts and prayers are with them at this extremely difficult time."