On 3 of my W-2 Forms, on the boxes below where it says “federal income tax withheld” its completely blank. Any reason as to why this may have happened?

Are they all from different employers and for different jobs?  Which boxes are blank?

Are these State Only copies?  

There federal.

Are they indeed three different companies or three copies from the same employer and NO copy shows any withholding?

If so, they did not withhold - Can you verify by looking at your pay stubs?  Did you file an exemption from withholding with the employer?  

Yes, all 3 of the W-2 forms come from 3 different employers but I did not work at these places for long. 

And for all 3 of the W-2 forms on the federal income tax withheld box are completely blank.

If box 2 federal income tax withholding is blank that means you didn't make enough to deduct federal taxes.  But you still might owe tax on the income.  The tax on your 1040 return is calculated on your total income for the year from all your jobs.  

Just enter all the W2s exactly as shown.