In a large, heatproof measuring cup, combine the vinegar, sugar, salt, mustard seeds, coriander seeds and dill seeds with the hot water and stir until the sugar and salt are dissolved. Let the brine cool.
In a large bowl, toss the cucumbers with the dill and garlic. Pour the brine over the cucumbers and turn to coat. Place a small plate over the cucumbers to keep them submerged, then cover the bowl with plastic wrap. Refrigerate the pickles overnight. Serve cold.
The pickles can be refrigerated in an airtight container for up to one week.