Not convinced that Didwot answer fully explains it. Try this. The three outputs on the secondary side of the transformer are likely to be the two ends of the tapping and a centre tap. This should give 11.4V between each of the ends and the centre. Try measuring the output between terminal 1 and 2, and then try between 2 and 3 with an AC voltmeter. If I am right you will get about 15V AC in each case (because the transformer is under no load conditions). You will also get about 30V if you measure between terminal 1 and 3. Do this and report back, because I am not sure you have a transformer adequate for the job. The 0.75 comments almost certainly refer to the current capacity of the transformer - 0.75Amps maximum. So you may have a 24V 0.75A transformer with a centre tapping. If so, this is capable of driving one 20W halogen bulb. How many bulbs are you hoping to drive, and are they 20W or 50W types?