i cannot download any new apps on my apple tv 3rd generation

that is correct, the 2g and 3g appletv did not have any storage to host apps, all their "apps" are apple server-side "channels"

if you want an app store get an appletv hd(4g) 4k(5g) 4kSE(4½g)

+1 The Apple TV 2 & 3 models do not have an app store, in order to get more apps you need an Apple TV HD or Apple TV 4K. I'd recommend the 4K.