Croatia or Vatican. I think Vatican agreed it to be Croatia. They will just give up the project and avoid further complications.
After all, it is better for all. No progressive human would like to see that freak project win. Project of forming new nations on the base of religion, at cost of genocide and cultural genocide on one complete Slavic people- Serbs, that are artificially divided on Orthodox, Catholic and Muslim parts, where Catholic and Muslim parts stimulated from outside work for foreign element and develop into new nations that steal heritage from original people- Serbs. Project that generate constant frictions in Europe.
and yet serbia has not joined the three seas initiative
Three Seas Initiative was a serious challenge to Poland-Serbia relations. It was first time in history that Poland openly moved hand into the Serbian realm with intention to grab. Fortunately, it was solved in time, on diplomatic level. Poland withdrew and than corrected things. Among Serbs this issue is explained in the sense - `well, Poland thought we died, killed by our mutual foes, but than Poland find out we are alive, kicking and than Poland apologized.`
What was the problem? Problem was that `Three Seas Initiative` includes Croatia, which is a part of Serbian realm, no matter its temporarily disputed. So when Poland moved in the region with `Three Seas Initiative` it was like Poland profit on Vatican`s genocide on Serbs, both- Orthodox and Catholic. Serbia did not react loudly. Instead, Serbia activated its connections within Poland and connections there Serbs have, with all large, small and tiny ethnic elements within Poland. Connections from time immemorial and primordial, older than state of Poland. Polonia responded by financing Serbian element within Bosnia. Kashubs, Lusatians responded by reviving connections with Balkan Serbs in mass media.
To give you picture- Poland`s move onto Croatia was like Serbia (hypothetically) moved onto Kracow, for example. Or even better example- onto region in Poland that was heavily exposed to Orthodoxy (and speaking hypothetically, tries to declare itself, sponsored from outside, a unique nation at the same time deny heritage of those who are still Polani, while only difference is religion). See? Croatia can`t be compared to Silesia, Kashubia or Lusatia? Its because Croatia represent essential Serbian realm, not some other ethos different to Serbs. Vatican and AH there experimented but process of engineering `Croat` nation was never finished.
So happened this meeting of Visegrad group > in Lany chateau, Czech Republic, on Thursday, October 3, 2019 >

Croatia was excluded.
As a side effect of that meeting, Poland and Visegrad fully opened their air-space to Serbia for any import of weaponry from Russia and Belarus. No complaints of EU and NATO had effect. No sanctions interrupted these arms transports. Poland`s air-space stay opened to Serbs and Polonia via companies of Corey Lewandowski pump money to Bosnian Serbs and it giving effects for the Slavic cause.
Import of weponry from Russia includes even S-400 Triumph


> youtube.com/watch?v=QM3Cfc0Wj7Y