iPhone 3G won't sync

Dear all,

I just received my iPhone 3G back from Apple Care, so now I have a 'new' phone.

When I connected it to iTunes I chose the last backup I had done before sending it to Apple.

Unfortunately the majority of my applications didn't sync, so now I can't even have Facebook, Instagram and WhatsApp on it.

I know that my iPhone 3G has been limited to iOS 4.2.1. I think this is the problem, because I continued to download apps updates and probably the versions which now I have on my computer don't fit my 3G anymore. However, if I go to the App Store and try to download them back, it says I need iOS 4.3.

My question is: is there a way to get older versions of my apps, the same ones I was using till two weeks ago?

Otherwise my iPhone is absolutely useless. Maybe from a past backup?

Thank you in advance for the help.

Best,

Moreno