To create a bundle of 3, you need 3 product groups.
For example, to create a bundle of jeans, t-shirts and shoes: Group 1 contains jeans, group 2 contains t-shirts, group 3 contains shoes.
However only 2 product groups are displayed by default, to add more group, please click "Add more product group" button or Plus icon:
If you want to remove 1 product group, click on X icon on this group