I don't use sequencers, so I'm not the best source of information on that. From what I understand (and hopefully someone will correct me if I'm wrong), Kronos is the only one of the three with a complete, multitrack, editable full length composition sequencer, if that's what you're looking for. I'm not saying it's best, or that it's simple, just that it does it. The Montage and the Fantom have pattern sequencers which let you build up compositions out of repeating patterns, which is highly suitable for some kinds of composition and completely unsuitable for others. But if it's suitable for you, it may provide more of the simplicity you're after, especially in conjunction with those boards' more modern looking interface. I think more often, "traditional" composition sequencing is being done on an an attached computer (Yamaha even provides free Cubasis for this purpose). Having the bigger screen and a mouse for editing probably makes that easier, and so the kinds of on-board sequencing that Roland and Yamaha have focussed on is a kind that an on-board interface can lend itself to. In this sense, Kronos is showing its age, in having a feature that was all the rage 10 and 20 years ago but has fallen out of favor as computer-based DAWs have gotten more capable and cheaper. At least that's my theory. But check out the manuals/videos for sequencing on these boards and see what you think. And maybe some others here with more sequencing experience will chime in as well.
Here's a good article on the Yamaha sequencer as of version 3.0:
https://www.yamahasynth.com/synths/mastering-montage-modx-pattern-sequencer-features-in-os-v3
The recently released 3.5 update adds some more features to the sequencer, which are covered here:
https://www.yamahasynth.com/synths/...rn-workflow-and-recording-features-in-os-v3-5
Here's a quick video overview on the Roland's sequencing:
And here's a guy who did a comparison video... jump to about 24 minutes to see his conclusion, but watch the video for more detail if you want...
Here's a Kronos video to start with...