An improvised explosive device killed the three service members Thursday, an alliance statement said, without providing nationalities or giving a specific location where the incident occurred.
In a bloody day for NATO troops in Afghanistan on Wednesday, insurgents killed six service members, including four who died in a single bomb blast in the volatile south of the country.
One service member was killed in the east in an insurgent attack and another died in the south in a separate roadside bombing - the weapon of choice for militants in countering a large-scale NATO-Afghan operation in the region.
2010 has been the deadliest year for international troops in the nine-year Afghan conflict. At least 37 NATO service members have been killed so far this month. More than 2,000 have died since the 2001 U.S.-led invasion.
Troop numbers have been ramped up in the past year to turn the screws on the insurgents. Fighting has intensified and casualties have mounted. The escalating toll has shaken the commitment of many NATO countries, with calls growing to start drawing down troops quickly.
The nearly 150,000 international troops and 220,000 Afghan government security forces - whose recruitment and training has been put on a fast track - are still struggling to gain the upper hand against an estimated 30,000 insurgents.