I don't see a point in Serbia sending it's army into Croatia in 1993. By that point most of the fighting had died down and UN troops were in the country. Croatia had managed to build up a defensive force of it's own and the Serb-held territory was mostly in the country's west. They also held land in eastern Croatia along the border with Serbia, but the Croatian military had no intention on attacking there as they assumed that a counterattack from Serbia itself would follow if they did so.
Montenegrin forces had already entered Croatia from the south in 1991 and failed to capture Dubrovnik, the only major city there.
In Bosnia they could help deal blows againt Muslim-held enclaves, but would very likely put an end to the conflict between Bosnian Croats and Muslims that was going on at the time as well.
At this point the Serbian government has little to win by directly attacking. It might provoke a western bombing campaign and further sanctions. They had already supplied the Serb rebels with plenty of the former Yugoslav People's Army's weaponry. There was a call-up before 1993 and the population was very much uninterested in being sent to Bosnia and Croatia. Those who did anwser the call were mosly nationalist extremist and by 1993 they were alredy fighting alongside the rebel local Serbs as volunteers. There's also the question of just how much fuel did Serbia's army even have in it's stock for such a level of mobilisation.