What do you mean by "west of Nairobi"? Is it Westlands or Nairobi West or Western Nairobi [which includes Kile-Lavi and Kawangware]? This is a very complicated City!
Any way; for a 3-yr-old, the best school will be one of the ones near your house. Visit all of them [they can't be that many] and make your own judgement. A good friend advised me never to take anyone's recommendation for a school. She said I should visit and see for myself.
It is amazing how much you can learn about a school by simply visiting - even if the staff don't talk to you....and that would be a lesson in itself!
Nothing is real unless it can be named; nothing has value unless it can be sold; money is worthless unless you spend it.