The ordnance exploded as the soldiers tried to dismantle it near the village of Aita al-Jabal near the border with Israel on Wednesday.
One of the soldiers was killed immediately and another was taken to a nearby hospital where he later died.
The third casualty was reported in critical condition in the same hospital, the security officials said.
The United Nations has confirmed that Israeli dropped nearly 400 cluster bombs across south Lebanon and says the bomblets have killed 14 people and wounded at least 50 since the truce.
The casualties take to six the number of deaths among Lebanese soldiers since they began deploying in the south on August 17 following a UN-brokered truce that ended 34-day conflict between Israel and Hezbollah.
Kofi Annan, the UN secretary general, has called on Israel to end its air and sea blockade of Lebanon.
Annan spoke in Ankara after talks on Turkey's contribution to a bigger UNIFIL peacekeeping force in Lebanon, as he continued attempts to broker a deal to end the eight-week blockade.
In a sign the embargo may be eroding, British Airways/BMED said it was resuming direct flights to Beirut after the British government gave assurances that it would be safe to do so.
Lebanon's Middle East Airlines and Royal Jordanian began flying regularly into the capital last month, but have complied with Israel's insistence that all such flights go via Amman.
Qatar Airways resumed direct flights to Beirut on Monday.
Gulf Air, which is co-owned by Bahrain and Oman, said on Wednesday that it would be resuming commercial flights to Lebanon from Saturday.