I would do this through UI scripting and the print dialog. You could set it to Layout > 2 pages per sheet which would get you 6 / page. Or you could UI script the handout layout dropdown. But it’s not exposed to AppleScript as far as I can tell. Which is typical of Keynote ApplesScript – there are a lot of little omissions that force UI scripting for some tasks.
Here’s how I have approached this: https://github.com/derickfay/keynote-to-pdf
Actually it looks like the Keynote print UI has changed a bit since I wrote that.
If you first create a print layout with 2 slides per page, and save it as 2x, this should work for you to print 6 slide / page handouts:
-- Keynote 6 version
tell application "System Events"
tell application process "Keynote"
set frontmost to true
repeat until window 1 exists
end repeat
-- Hide Inspector if visible
if menu item "Hide Inspector" of menu 1 of menu bar item "View" of menu bar 1 exists then
keystroke "i" using {command down, option down}
end if
-- Print; wait until the sheet is visible
click menu item "Print…" of menu 1 of menu bar item "File" of menu bar 1
repeat until sheet 1 of window 1 exists
end repeat
set thePopUp to first pop up button of sheet 1 of window 1 whose description is "Presets"
click thePopUp
click menu item "2x" of menu 1 of thePopUp --replace if desired with your preferred preset
if value of (radio button "Selection" of sheet 1 of window 1) is 0 then
click (radio button "Selection" of sheet 1 of window 1)
end if --set to print selected slides only -- you can use the same technique to adjust other checkboxes as desired
click checkbox 5 of sheet 1 of window 1 -- replace to suit your needs: 5 = Handout
click menu button "PDF" of sheet 1 of window 1
click menu item "Save as PDF…" of menu 1 of menu button "PDF" of sheet 1 of window 1 -- Save as PDF...
end tell
end tell