I disagree. Nothing says you must break the shrinkwrap on the other two boxes. All three boxes are identical - I don't believe Word 5.1 uses serial codes at all. You possess three legitimate boxes, and therefore have three licenses. You can't sell the shrinkwrapped boxes as long as you have Word 5.1 installed on three computers. In the future though, you could erase Word 5.1 from one of the computers and then sell one of the shrinkwrapped boxes. You are still upholding the basic ideal which you hold - one copy for each installation you have - you're just maintaining some value in those other two boxes.
This is how I was able to obtain about 8 copies of Microsoft Project 4.0 for Mac sealed and 2 copies of Microsoft Office 98 sealed. The company which previously had the software had only opened a few of the boxes. Project 4.0 had no serial codes so this worked fine, although Office 98 has serial codes and so they were using codes more than once, which was wrong.
Now, if Word 5.1 used serial codes, then yes, to remain legitimate, you would need to open all three boxes to obtain a unique code for each installation.
Anyway, it's just a recommendation.
They'll only increase in scarcity over time.