Yugoslavia was a mixture of central socialism and free market, which often seemed to consist of multiple state agencies competing with one another. This is what happened with their airlines.
JAT was the official state airline, and the separate Air Jugoslavia was their charter arm but only on the commercial side; operationally they just used JAT aircraft although with different flight number prefixes.
Aviogenex was a subsidiary of the state-owned General Export Agency (hence the name), who favoured Tupolev 134s as a cross-arrangement for the substantial amount of agricultural produce they also exported to the Soviet Union.
Inex-Adria, as it was known at the time, was part of InterExport, yet another state agency charged with managing Imports and Exports (again hence the name), with a mainly DC9 fleet.
Tourism was a principal foreign currency earner, what these businesses were about, and they also had access to foreign currency funds for aircraft purchases needed to support this.