Suggest you stop confusing users by having mail merge fields called <Business Line 1>, <Business Line 2> in the default templates.
Not sure when this started, (or even, how it works?) but the fields on my screen are labelled Address 1, Address 2 etc , not Business Line 1, Business Line 2, so how yould you expect users to assume they are tthe same? And, of course, if you wish to add <Business Line 3> to a standard letter template from the Add Mail Merge Fields window you find it isn't there.
It's complicated enough without deliberately making it more so ! ;-)