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.
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".
Gates welcomed a pledge by Italy to lift restrictions on the movement of its 2,350 soldiers, most stationed in western Afghanistan, but it remained unclear whether Rome would be willing to move them to a more hostile area.
Isaf now comprises almost 53,000 troops from 40 nations, up from 33,000 troops 18 months ago, but commanders continue to press for more help.
France has agreed to send 700 additional troops, which will deploy later this year in an area near Kabul, Afghanistan's capital, in eastern Afghanistan.
This will free up around 1,000 US soldiers to move into the more violent south.
Around 3,500 US marines have also been deployed but they will leave by November.
Isaf has said it is trying to spread the influence of the weak central government across the country.