but my s3 nothing has downloaded, is this a nexus only ?

In my experience this has been hit and miss for me.

I have upgraded from HTC Desire to a Samsung Galaxy S2 and done several s/w upgrades on each device plus a few hard resets over the years, sometimes the apps come down and sometimes they don't

I have always put it down that l have multiple google accounts on my phone.

i do have 2 accounts but 99% are all on one of the accounts

I aleays turn this off on my Nexus as it can cause problems. OK on Touchwiz phones though.

I got a Galaxy SIII yesterday,(moving from a Nexus S) and the guy in CPW downloaded nearly all of my apps (and contacts list) that I had installed on the Nexus presumably from Google cloud or whatever. I did have to download a few that where missing, but apart from that, it was fine.