**Wheat, rice, and maize** are the three crops that contribute the maximum to global food grain production. These crops are staple foods for a large portion of the world's population and are vital for ensuring food security.

**1. Wheat**
- Wheat is one of the most widely cultivated and consumed crops globally, especially in temperate regions.
- It is a staple food for millions of people and is used to make various products like bread, pasta, and pastries.
- Wheat is adaptable to different climates and can be grown in a variety of soil types.
- Major wheat-producing countries include China, India, Russia, and the United States.

**2. Rice**
- Rice is the primary staple food for more than half of the world's population, especially in Asian countries.
- It is a versatile crop that can be grown in both flooded and rainfed conditions.
- Rice provides a significant source of carbohydrates and essential nutrients.
- Major rice-producing countries include China, India, Indonesia, and Bangladesh.

**3. Maize (Corn)**
- Maize is a crucial crop for both human consumption and animal feed.
- It is widely grown in various regions due to its adaptability to different climates and soil types.
- Maize is used in the production of numerous food products, such as cornmeal, corn oil, and corn syrup.
- Major maize-producing countries include the United States, China, Brazil, and Argentina.

These three crops have high yields, are versatile in their uses, and are well-suited to different growing conditions. They play a vital role in meeting global food demand and ensuring food security for millions of people.

It is important to note that crop production and contribution to global food grain production can vary over time due to various factors such as climate change, technological advancements, and shifts in agricultural practices. However, as of 2005, wheat, rice, and maize were the top three crops contributing to global food grain production.