Filler flowers in Japanese are known as

Filler flowers in Japanese are known as Jushi.
Jushi is any flowers or leaves that do not make up the 3 main placements. They are meant to complement the shin, soe and tai. Add as many jushi as you like, but only in odd numbers.
Soe is two-thirds of the shin, and tai is two-thirds of soe. If shin and soe are branches, accompanying decorative flowers, called jushi, will be half as long as shin, soe and tai.