Bring the sugar, mustard seeds, celery seeds, allspice, turmeric, garlic, onion, 3 teaspoons salt and 1 cup water to a boil in a small saucepan. Place the cucumbers in medium bowl. Pour the hot spice mixture over the cucumbers, then stir in the vinegar. Let cool to room temperature, about 30 minutes. Serve, or refrigerate in an airtight container for up to 2 weeks.