I've heard that license conditions must be applied if only I want to distribute my app. But, if I have a frontend and I use GPL3 licensed library on it, does that count as distributed? Do I need to make my frontend disclose source?
You have written a program de novo, except that it uses a third-party library which has been made available to you under the terms of the GPLv3. You want to know if distributing your program, linked to a copy of the library, counts as distribution of the library, and what it means if it does.
Yes, it does count as distribution of the library.
Do I need to make my frontend disclose source?
You do have some obligations, but they do not (strictly) include that one. Firstly, you will have all the GPLv3 s4 and s6 obligations in respect of the library.
Whether you have the same obligations with respect to your code depends on whether the resulting binary is a derivative work of the library, in copyright terms. This is not a settled question; we have a question that summarises why it should be, and one which summarises why it shouldn't be. Like the FSF, I hold that it should be, and is.
If this is so, then you have similar obligations (ss 5 and 6) in respect of your code, which include: it must be distributed under GPLv3, and the source must be available (also under GPLv3) to people who get copies of the binary. The program doesn't have to arrange for this itself; you can distribute source alongside the applications, or you can give it out on demand; your options are laid out in GPLv3 s6.