Jordan (15 seasons): 6 championships, 6 Finals MVPs, 5 MVPs, ROY, DPOY, 10 1st team All NBA selections, and 9 1st team All Defensive selections; averaged 30.1 ppg for his career, which is #1 in NBA history
Leonard (8 seasons and counting): 2 championships, 2 Finals MVPs, 0 MVPs, 2 DPOYs, 2 1st team All NBA selections, and 3 1st team All Defensive selections; averaged 17.7 ppg in career to date
No matter what Leonard does this season, it does not put him in any serious conversation for GOAT. If he has 5 more dominant seasons, maybe it could happen, but they would have to be truly dominant, with a lot of earned honors/awards in addition to more championships. He has a long way to go just to crack the top 25 players of all-time, much less get into the GOAT conversation.
Leading 3 franchises to championships hasn't been done because we have only just reached this era of frequent movement of elite players between franchises. Someone will be the first to do it, but that by itself doesn't equate to GOAT discussion.
And, despite winning Finals MVP in 2013-14, Duncan was still the better player in that playoff run, averaging more points on better shooting, more rebounds, more assists, more blocks, and fewer turnovers. This past season was the only one in which Leonard was the alpha player on a championship team.