Convert APK to 3GP

Android installation packages (.apk) generally do not contain documents, pictures, or any multimedia file so it is not possible to convert apk to 3gp. It might be possible that you were looking for an installation package for an Android app converter. In such case, you can either try to find one using a proper conversion query like 3GP to MP3, or browser Google Play to find the app you were looking for.

In any case, .apk file is essentially renamed .zip file, so you can extract the content and check for the 3gp file you are looking for, but chances for this are pretty slim to be honest.

Simply put, it is impossible to convert APK to 3GP directly.
Thus, there is no so-called apk to 3gp converter or a free online .apk to .3gp converter to be found.