I've deleted apps from my iTunes.
Can I transfer them somehow from my iPhone? Or I should re-download them one by one?
When I try to sync my iPhone, iTunes want's to delete apps…
If you right click the iPhone in the list on the left there should be an option to transfer purchases. Click that and it should transfer all your apps to your computer.
Yes, you can transfer the apps back to iPhone. But since iTunes only allows you to transfer contents from iTunes to iPhone, you may have to use a 3rd party software.
To find out more about how to transfer apps from iphone to PC, you can refer to this article.