3:15 viisitoista yli kolme = fifteen over three.
3:30 puoli neljä = half four.
3:45 viisitoista vaille neljä = fifteen short of four.
We also use 24 hour clocks but if the dinner is at 17:30, it will just be said to be at half six and you figure am/pm out of context - if it's ambiguous, we say "six in the morning / six in the evening".