Serbian President Aleksandar Vucic, Albanian Prime Minister Edi Rama and his North Macedonian counterpart Zoran Zaev want to promote a regrouping of countries in the divided region to achieve "four basic freedoms -- free movement of goods, services and capital as well as free movement of people," said a joint declaration after their meeting in Novi Sad, in northern Serbia.
They plan to include in the initiative also Bosnia, Montenegro and Kosovo.
The latter, Serbia's former ethnic Albanian-majority province, is still not recognised by Belgrade.
The issue remains the most sensitive one in the Balkans, the region torn apart in a series of 1990s wars that accompanied the collapse of Yugoslavia.
"The fact that we don't have the same view over Kosovo ... has nothing to do with free movement of goods and persons," Vucic told reporters when asked whether Kosovo was invited to join the initiative.
"Our goal is that by 2021 it becomes possible to cross borders between our countries with an ordinary ID card ... to create an unified job market," he said.
The leaders acknowledged that their initiative is based on the "four freedoms" that underpin the European Union's common market: the free movement of goods, capital, services and labour.
Serbia opened EU membership talks in 2014, while Albania and North Macedonia are hoping to launch them later this year.
"We can no longer wait for someone from the outside to do something for us ... 2021 is the date by which we want to make from this region an open region," Rama said.
Albania, Bosnia, Kosovo, Montenegro, North Macedonia and Serbia have a combined population of nearly 20 million people.
Zaev added that with the initiative the three countries wanted to send a message that the Balkans are no longer a "powder keg, but a stable region dedicated to economic development and European integration".
The next trilateral meeting should be held on November 10, in North Macedonia, when first concrete deals would be signed.