Would an iphone 3g have newer apps?

Since it's an older phone, would newer apps still show up in google play store or would it be older apps?

They should still show up regardless which iphone you have. The app should say what it "is" compatible with. Your best chance is to to keep it updated with the latest IOS.

That is depending on the iOS version of the 3g, I'm not sure if you can upgrade to the iOS 7 even if you do i bet it will be laggy but you can still get the newer apps if you upgrade to iOS 7. Otherwise i'm not 100% sure sorry. It's not google play btw.

Google Play is the name of the store for Android phones, not iPhones which use the App Store.

To answer the question most apps would not be able to run on an iPhone 3G, at least their latest versions. Apple recently introduced an option that allows you to download the last working versions of apps to use on older models.