While it is not uncommon to say "at three" instead of "at fifteen" in German, this only ever happens when context sufficiently avoids ambiguity. The language does not even know a generic qualifier for 12-based times (like am/pm), you would have to use the correct time of day name (non/afternoon/evening) for disambiguation. Much easier (and therefore much more common) to say "fifteen" instead. (for the second half, first half can only be specified the complicated way)