BY TAREK AMARA and JIHED ABIDELLAOUI THREE years of drought have dried up Tunisian reservoirs, threatening harvests that are critical to the North African country's battered economy and pushing the government to raise tap water prices for homes and businesses. Since September only 110 million cubic meters of rain fell in Tunisia, about a fifth of the normal rate, and officials in the farmers union and the main trade union warned that grain crops will suffer - adding to existing problems of food supply. "The situation is very dangerous because of years of continuous drought," said Hammadi Habib, an Agriculture…