Designing a Buck Boost Regulator 12V Circuit
I could decrease/increase the input voltage and combine that with a
  boost or buck/linear regulator to achieve a 12V output but I would
  prefer to keep the input as 12V.


Well, that's what a buck-boost regulator does so, when you say this in the next paragraph...


  I found this post (Stabilize 12V to 12V) which recommends a Buck-Boost
  Converter. This seems like it would work for my application.


You are inadvertently forming a contradiction (no problem, just saying).


  I've had a look on RS components through some datasheets but the
  typical circuit applications either show the IC in Buck or Boost
  configuration and not as a Buck-Boost.


Well, they do have a section called buck boost: -



Try this link to the above page and note that top of the list is the MC34063 (similar part number to what you referred to) and might be very similar but make by ON semiconductor rather than TI but, do read the data sheets carefully.


  For example, some ICs seem to mention PWM control, I assume this would
  mean I need to include something like a 555 IC to control the
  buck/boost functionality?


No, that won't be needed - buck-boost chips will have this built inside for controlling the transistors associated with the circuitry.


  is there a particular type that would be best suited to this
  application?


Generally I don't use RS or FEC to search for what I want, I got direct to TI or AD and use there search engines. For example this one by AD (Analog Devices): -



Press "search" and it comes up with 13 options: -



I browsed down that list and half-heartedly chose the device that appeared to use the fewest components: -



But I could have gone to choose a device that used internal transistors (if there were one that would do 3 amps) and find just one suitable device: -



If you need any more help choosing just say.