It's a bit tricky to say there are only three main jobs of proteins, as they are incredibly diverse and play vital roles in almost every aspect of life. However, we can categorize their functions into three broad categories:

1. Structural Support: Proteins provide the framework and support for cells, tissues, and organs. Examples include:

* Collagen: A fibrous protein found in skin, bones, tendons, and cartilage, giving them strength and flexibility.

* Keratin: A protein found in hair, nails, and skin, providing protection and structure.

2. Enzymes: These proteins act as catalysts, speeding up chemical reactions within the body. They are essential for everything from digesting food to building new molecules. Examples include:

* Lactase: An enzyme that breaks down lactose in milk.

* DNA polymerase: An enzyme that copies DNA during cell division.

3. Signaling and Transport: Proteins play a crucial role in cell communication and the movement of molecules throughout the body. Examples include:

* Hormones: Proteins like insulin and growth hormone act as chemical messengers, regulating various bodily functions.

* Antibodies: Proteins that fight infections by binding to and neutralizing pathogens.

* Hemoglobin: A protein in red blood cells that carries oxygen throughout the body.

While these are the three most prominent categories, proteins have many other essential functions, including:

* Regulation of gene expression

* Cell-cell adhesion

* Defense against pathogens

* Energy storage

So, while these three categories provide a good starting point, it's important to remember the vast array of functions proteins perform.