irb(main):001:0> a,b,c = [1,2],3
=> [[1, 2], 3]
irb(main):002:0> a
=> [1, 2]
irb(main):003:0> b
=> 3
irb(main):004:0> c
=> nil
irb(main):005:0> a,b,c = [[1,2],3]
=> [[1, 2], 3]
irb(main):006:0> a
=> [1, 2]
irb(main):007:0> b
=> 3
irb(main):008:0> c
=> nil

これって、どのように理解したらいいですかね?