Question

Once the delegates determined the three fifths compromise what issue did they tackle next?

Answer 1

Once the three-fifths compromise was determined by the delegates, the issue they tackled next was the Presidential Election process.

Answer 2

I believe the answer is: Presidential election process

At that time, the south argue that conducting nation wide election would not benefited the south because they have lower population compared to the north.
Because this, three fifth compromise was created to count 3/5 of the slave population in the north as eligible voters. This would give the southern representative better chance to win the presidential election.