Throw Me Away, Bring Me In

How many seconds are there in one year?

Think of a number

Step 1: Multiply it by 3

Step 2: Add 6

Step 3: Divide by 3

Step 4: Subtract the number chosen in step one from the answer in step 4.

What is the answer?

What goes up but never comes down?

What am I?

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