The difference between some of those conventions and now is the primary system. The contested conventions before the 20th century did not involve primary elections. It is one thing to decide a nominee based solely on conventioneer votes, another to disavow the state primary voters’ wishes and pull some new candidate out of their orifices and foist him on the electorate.
One of the three candidates remaining in the race will be the nominee. It’s fiction that a fourth person will drop in and be nominated (e.g., Ryan). If it’s a contested convention, the three will battle it out, and that’s not bad, it’s quintessential American politics. All the primaries & caucuses that preceded are to *select* the convention delegates who will *elect* the nominee. And each state & territory gets to determine how delegates are selected. Federalism at the party level - good!