BERLIN (Reuters) - Angela Merkel s conservatives maintained a strong lead over their rival Social Democrats (SPD), who lost ground slightly, in a poll published on Thursday, three days before an election the German chancellor is set to win. The Forschungsgruppe Wahlen poll for broadcaster ZDF showed Merkel s CDU/CSU bloc was unchanged on 36 percent while the SPD dropped by 1.5 points to 21.5 percent. The anti-immigrant Alternative for Germany (AfD), which is expected to become the first far-right party in parliament for more than half a century, looked set to come third with 11 percent - one point more than last week. The pro-business Free Democrats were on 10 percent, followed by the radical Left on 8.5 percent and the Greens on 8 percent. That would present as potential alliance options a  grand coalition  between Merkel s conservatives and the SPD, or a three-way  Jamaica  alliance of the conservatives, FDP and Greens - so named because the colours of the parties match those of the Caribbean island s flag. In a theoretical direct vote for chancellor, Merkel would win 56 percent while her SPD challenger Martin Schulz would score 32 percent, according to the poll of 1,725 people conducted on Sept. 20 and 21. 