why not 3-12's for 36 volt lithium?

[john fox]

I need to start thinking about changing out my 6yr. old battery pack in my 1998 txt.
thinking about going lithium.
I thought 3-12v batteries would be more versatile than 1-36v battery but read in another thread it's better to go 1-36v WHY?
would there be a problem with three bms as apposed to only one?
the reason I was thinking 3-12 would be better was if in the future if I had to replace my old cart with a newer one {48 v } my three 12v lithium batteries could move easily to a solar system where one 36v battery maybe not.
thoughts?