3 sentences

1. 这完全是悠悠之谈，不足为信。

   zhè wán​quán shì yōu​yōu zhī tán, bù​zú wéi xìn.

   This is completely baseless talk and is not to be believed.

   Source: Dong Chinese Dictionary

2. 这种事见多了也就不足为怪了。

   zhèi zhǒng shì jiàn duō le yě jiù bù​zú wéi guài le.

   Once you've seen enough of this kind of thing, it's no longer surprising.

   Source: Dong Chinese Dictionary

3. 由于证据不全，他的证词不足为凭。

   yóu​yú zhèng​jù bù quán, tā de zhèng​cí bù​zú wéi píng.

   Due to incomplete evidence, his testimony cannot be relied upon.

   Source: Dong Chinese Dictionary