A trimerous flower is the one which 

A trimerous flower is the one which has floral appendages in multiples of three.

A trimerous flower is a type of flower where the floral parts, such as petals, sepals, stamens, or carpels, are arranged in multiples of three. This means that each floral whorl contains three or a multiple of three parts. Trimerous flowers are commonly found in monocot plants, such as lilies and grasses. This arrangement is a key characteristic that helps distinguish monocots from dicots, which typically have floral parts arranged in multiples of four or five. Trimerous flowers are an adaptation seen in specific plant families to support their reproductive processes effectively.