Three litres is less than one US gallon, and that's too small for almost any aquarium fish. The female guppy could have died from the shock of moving to a new habitat or from ammonia spikes caused by uneaten food in the small aquarium. If she had dropped the fry as soon as she was put into the little tank, the fry may have survived for a while, provided there were plenty of live, bushy plants. In any case, the angelfish will soon outgrow the larger aquarium and will prey on any smaller fish. Maybe the best idea is to take the angelfish back to the pet shop and concentrate on raising guppies. Guppies are still one of the most attractive aquarium fish available, and they're fairly easy to keep. The alternative is to have a house full of aquariums like I do; some people will think you're a nut.