I assume these three days fall in the middle of your holiday so you will already be in Florida?

I would quote them the full 80 hours plus your expenses. As you are already in the US it will still compare favourably with flying one of your clients employees from the UK (£4k flight v prob £1k on AA?). If you spin a sob-story about having to break your holiday then they should see that the 80 hours of billing is justified.