A multinational force spokesman said the soldiers died in an improvised bomb explosion.
Their nationalities were not disclosed.
The latest casualties brought to 83 the number of foreign soldiers killed this year in Afghanistan, where about 70,000 international troops are fighting a Taliban anti-government campaign.
Two British soldiers and a Romanian trooper - all serving with the separate Nato-led International Security Assistance Force (Isaf) - were killed in two attacks in Afghanistan on Thursday and Friday.
Gates's appeal
The bomb attack in Farah comes one day after Robert Gates, the US defence secretary, said that for the first time, the monthly total of American and allied combat deaths in Afghanistan exceeded the toll in Iraq during May.
He urged Nato allies to live up to past pledges of troops and equipment to Afghanistan.
After talks with Nato defence ministers in Brussels, Belgium, Gates said: "I expect government decisions and actions to match government rhetoric."
He said that despite vows at a Nato summit in Bucharest in April to raise more troop contributions, "many of the same shortfalls that existed 18 months ago still exist today".