@ehd, you are making a fundamental mistake. You should not have separate files for different years. Manager is a perpetual system that keeps all your records in a single file. Otherwise, the permanent accounts (the balance sheet accounts) will not be correct. If you are going through the old-fashioned process of closing accounts and then re-opening books in the new accounting year, you are making a huge amount of extra work for yourself. All you need to do is reset the Summary period for the new year and create new reports for those you use (which you would have to create doing it the hard way, too). This way, accounts payable and receivable carry over without the need to re-enter invoices, etc. And you can create comparative year-to-year reports.
You can read more about this in these topics: Unclear on Year-end procedures and Reset Cash & Bank for new year.
The actual problem you report comes from the fact that you cannot open Manager data files directly. They are called by the program, but the program does not know where to look for them. You should be able to open them by importing them as a backup using the Add Business button and navigating to them. This will create a different business in your existing Manager folder.
I don’t know whether there is any way to merge the three years of data back into one proper file. You will have to ask @lubos about that.
OF COURSE, BACK UP YOUR CURRENT ACCOUNTS AT A SEPARATE LOCATION BEFORE DOING ANYTHING ELSE.