Hi I had the same problem, I update to iTunes 12.3.3.17, rebooted my PC and no access to iTunes app updates. Here is were what I did to fix this for me I hope it helps some of you out.
I went back to iTunes, I signed out of iTunes by going to the "Store" menu and choosing the option "Sign Out"
Next I signed back in again by going back to the "Store" menu in iTunes and chose to "Sign In". I immediately tried to download the apps, but still no luck, that took about 10 seconds, not sure if this part matters, but perhaps giving iTunes sometime to process the information I gave it helps.
Anyways last but not least, I closed iTunes, waited 3 seconds then relaunched it, and immediately I was able to see and download my app updates.
Just an FYI I am running Windows 7 64Bit Ultimate, maybe that detail might help
Cheers I hope this fixes the problem for you.