I am a Five Letter Word

Which 3-letter word can be inserted into all four lines above to form complete words?

P_ _ _ N T

D E _ _ _ S T

S T _ _ _

C _ _ _ E R

What occurs:

Twice in a Week, Once in a Year!

But never in a Day?

What goes up and never comes down?

I have branches, but no fruit, trunk or leaves. 

What am I?

Figure out how many trailing zeroes are in 1000!