Bulgarian Prime Minister Boyko Borisov said the three countries were not prepared to become a "buffer zone" for the thousands of people trying to reach the European Union, many fleeing war in Syria and Afghanistan.
The EU is holding a mini-summit in Brussels tomorrow to try to work out a cross-border approach to the worst migrant crisis in Europe since World War II, which has seen 670,000 people land this year.
Borisov warned that if other EU countries followed suit, Bulgaria, Romania and Serbia would have to act.
"All three countries, we are ready if Germany and Austria and other countries close their borders (...) we will be ready to also close our borders at that very same moment," Borisov said after talks with Romanian and Serbian leaders in Sofia.
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"We will not let our nations become a buffer zone of the migrant flows that will become stranded between Turkey and the fences built up from Serbia."
"If there are countries that shut their borders and build fences, we also have the right to act timely and protect our countries. But this is not a good decision," Ponta said.
Over the past months non-EU member Serbia has been swamped by migrants on their way from Greece and Macedonia to northern Europe, though Bulgaria and Romania have so far remained on the sidelines of the influx.
Juncker has drafted 16 proposals for tomorrow's summit, including an undertaking that no country will let migrants through to an adjoining state without first getting the neighbour's agreement to do so.