It's hard to pick just three helpful bacteria as there are many with beneficial roles! However, here are three important examples that demonstrate different kinds of helpfulness:

1. Lactobacillus: These are a group of bacteria found in fermented foods like yogurt, sauerkraut, and kimchi. They contribute to digestion by breaking down lactose (milk sugar) and producing lactic acid, which inhibits harmful bacteria in the gut.

2. Bifidobacterium: These bacteria also reside in the gut and are particularly important for infants, aiding in digestion and nutrient absorption. They contribute to overall gut health and may have roles in immune system development.

3. Nitrobacter: These bacteria play a crucial role in the nitrogen cycle, a fundamental process for life on Earth. They convert nitrite into nitrate, making nitrogen available to plants and helping to maintain healthy soil ecosystems.

It's important to remember that there are many other helpful bacteria, each playing a vital role in various ecosystems, industries, and our own bodies!