There are two parts to the K-3 - the petition and the visa process. Once you are married to a US citizen and file an I-130 Petition, you can then file for the K-3 petition in the US. The K-3 petition is filed in the US always. The K-3 VISA is then processed in the foreign person's home country (and if couple married abroad, must be process in the country where the marriage took place (which may be different than the foreign national's home country). So, if married in Germany, the K-3 visa would be processed there.