ДЕМО
(свойство flex-basis)

basis:auto
basis:auto
basis:auto

basis: 0
basis: 0
basis: 0

basis: 0
basis:auto
basis:auto

basis: 300px
basis:auto
basis:auto

basis: 300px
basis: 50%
basis: 50%

basis: 50%
basis:auto
basis:auto

basis: 50%
basis: 50%
basis: 300px

basis: 30%
basis: 30%
basis: 30%

basis: 33.333%
basis: 33.333%
basis: 33.333%