Excel File Sharing

Hey all!
I'm looking for a sharing solution that could work efficiently on a particular excel file which is right now being used by 3 operators to punch data. However, they all enter data separately. Each one has been allotted specific time slots to work on the file and they have to finish their jobs within their respective time slots or request for an extension from the colleagues if necessary. So its not even being SHARED! I just dont know how to term this so apologies if i'm tagging the thread wrong.

However, now I need to start working on that file too. But lets just say I cant fit this additional work on my desk hours and i'll have to finish the task from home or at move. My Company however, does not have its own server but it has access to broadband internet. (512kb line)

I know "Workbook Sharing" isnt very effective so i'm looking on an excel workbook backup route, with a few restrictions.

The office computers go offline after office hours. I have a backup software that can make backup copies of the file and upload it to wherever i want on the internet or even send them as an email (GFI Backup, free edition).

Now i see 3 situations where I'll have to work on:

1. I'm working from from my laptop and at the same time another operator is working in the office. (Office Working Hours)
2. I'm working from my laptop and the office is shut. (Office after-hours)
3. Operator wants to work on the sheet but the latest modified file is on my laptop. (Early morning next day)

2 and 3 sort of solve on their own using the Backup software, i'll just have find a way to make sure the file is not forgotten to be downloaded from the email/internet before starting work on it. (Please help me on this too)

However, 1 is a problem, how do i make sure when i'm working on the file during office hours nobody is working on that same file? I need the READ-ONLY message for this...

Or should i look at some better more efficient way at handling this?

Thanks and Regards
Mohit