Changed loan terms, what did I lose?

Hi, first time posting here, great forum BTW.

I have been negotiating a relocation package with my company and they originally agreed to provide an interest free 5 year loan for $45,000 to be paid back by me in equal amounts annually.

The terms have now changed to 3 years and I'd like to know the value of the money I've lost due to this change.

Basically over the 3 years I will be short $6000 each year in comparison to the 5 year loan. So from my perspective I will be short $18000 plus interest at the end of 3 years. How do I calculate this interest? Would I just take the future value of the $18k at, say 6.0% interest/yr? Should this be compounded?

Thank you in advance for your assistance!