flo, flu=流れる
superfluous / あふれて流れる→不要な、余分な
cf. She worked so well that my help was superfluous.
fluctuate / 流れる→変動する、同様する
cf. During the crisis, oil prices fluctuated between $20 and $40 a barrel.
fluent / 流れるような→流暢さ
cf. Applicants should be fluent in German.