Set the display precision of a float in Ruby
z.round(2) or x.round(3) is the simplest solution. See http://www.ruby-doc.org/core-1.9.3/Float.html#method-i-round.
That said, that will only ensure that it is no more than that many digits. In the case of 1/3 that is fine, but if you had say 0.25.round(3) you will get 0.25, not 0.250.
I would normally just do the conversion in open code, something like:
puts "%5.2f" % [1.0/3.0] Ruby calls Kernel#format for expressions like this, because String has a core operator % defined on it. Think of it as printf for Ruby if that rings any bells for you.