This sentence...

Somehow something clicks wrongly to me here.

(three people walked in)
That was more people checking in than in the whole last week.

 

Whole is more about something being whole, undivided.

Try entire, I think that will clear your wrongness up, also

That was more people checking in than we had the entire past week.

 

Indeed entire's better. But I'm not speaking for the hotel, so we is impossible.

- That was more people checking in than the entire past week. -

Sounds good. Thanks.

 

Three people walked in, more customers than the whole week combined.

I think the "than in" jars the pace a little.

 

Quick fix:

That was more people checking in than in the whole of last week.

 

Ditto John Eff - my first instinct was to write "the whole of last week".

"The whole last week" just sounds wrong - it could certainly be something you'd say but definitely not write, unless of course your narrative is done in first person.

 

Three new guests checked in that day--more than had shown up in a week.

 

Three people arrived to check in--more than all of last week combined.

 

^- This.

/agree

It's appropriate and is not cumbersome. Other ways of conveying the same information may not flow as well, depending on the style of the writing.