Cambridge stage to American system correlation

Does anyone happen to know which stage a third grader and a first grader would be be on when switching from American system to Cambridge System of education? 

Would an American third grader on par with grade level knowledge be behind, ahead, or just right with a student in the third grade or equivalent class?  Which class would an American third grader go into in Jeddah?  (actually son is going 'into' third grade next year and I'm trying to see where he might be in it. I also have a Kindy kid and he'll be going into First grade but I'm unsure of the grade level there in Jeddah.

Anyone have experience with this?

in jeddah, there are schools following British syllabus and there are schools following American syllabus.
the famous ones are Jeddah prep and grammar school, British international school, Thamrer school, Yusr international school etc.