Give any three ways in which bacteria are useful to us .

A large number of bacteria are actually helpful to us. Here are some examples:
Making yogurt: Lactobacilli are the bacteria that help us in making yogurt from milk. When these bacteria are added to the milk, they break down the milk sugar and generate lactic acid. The lactic acid, in turn, makes the milk curdle, and we get yogurt. Streptococcus thermophilus is another bacterium that can be used for making yogurt.
Soil bacteria: There are some bacteria that can help in "nitrogen fixation." In this process, the bacteria (such as Azobacter, Rhizobium, etc.), convert atmospheric nitrogen to a form that can be easily absorbed by the plants. Thus, these bacteria help provide nitrogen naturally and reduce the necessity of using fertilizers.
Waste management: A large number of bacteria help us in minimizing the organic waste that is generated in our industries and homes. For example, bacteria help in reduction of the organic content of wastewater and thus help treat it. Bacteria can also break down organic solid waste and thus help us in managing it.
Hope this helps.