M B Abbasi, Pakistan's ambassador to Iran, said the meeting was a first for the three countries.
"We are making history, as it is the first time that Iran is being equally integrated with the issues that Pakistan and Afghanistan are facing," Abbasi said.
Zardari proposed the trilateral summit after meeting Ahmadinejad and Karzai in talks in early March that focused on rebuilding war-shattered Afghanistan.
The meeting comes as Pakistani army troops battle Taliban fighters in the northwest of the country, close to the border with Afghanistan.
Haroun Mir, the deputy director of Afghanistan's Centre for Research and Policy Studies, said that Ahmadinejad's decision to host the meeting is an attempt by Tehran to show it has influence in the region.
"I think it is a signal from the Iranians to the Americans that they are ready to play their cards and use their influence to stabilise Afghanistan," Mir told Al Jazeera.
But Mir said that Iranian-American relations will also play an integral role in promoting stability across the region.
"If the American-Iranian relationship could improve I think it would have a positive impact in the region for Pakistan and Afghanistan," he said.
Zardari is also expected to use the summit to focus on increasing co-operation between the three countries on trade and economic initiatives.
The summit comes a day after Afghan troops used air raids to destroy 92 tonnes of drugs, heroin-processing chemicals and bomb-making materials, in the largest counter-narcotics operation to date in the country.
At least 60 suspected fighters were killed in the operation on Saturday in the southern Afghan province of Helmand, a key stronghold of the Taliban.
Afghanistan is home to 90 per cent of the world's opium, most of which is converted into heroin within the country and smuggled out through Iran and Pakistan.
Pakistan, which faces a severe electricity supply crisis, is also expected to discuss a proposal for 1000 megawatts of power to be supplied from Iran to Pakistan via Baluchistan.
"I think Pakistanis are in need of energy and Iran is the country with abundant sources of energy. This is a leverage that Iran could play," Mir said.