THREE RIDDLES

1. What can be either a welcome or a warning?

2. What's a milliHelen?

3)
A hundred and one
By fifty divide,
And if a cipher
Is rightly applied,
The answer is one from nine.

Answers:

1. A dog's bark 

2.  The time it takes to launch one ship
(from Helen of Troy - "the face that launched a thousand ships")

3. CLIO  (one of the Nine Muses)