Google wants the capacity to do really big acquisitions (like Facebook big). Assuming Google did decide they wanted to buy facebook for $70B, they would need to raise some serious cash. Much of Google's cash is in overseas accounts and would incur US income taxes if repatriated.
This $3B is a practice round. The $50M or so in wasted interest expense is the cost of an option to do a huge acquisition.
Could it also be a case of Google's financial planners thinking they "don't have enough debt" in the eyes of investors?
If their debt to equity balance sheet is too tilted, they could be perceived as 'lazy'. This move could be a way to acquire more debt that is easy to manage, as well as improve their credit rating (if they need it).
In some ways, I wonder if this is a move to boost their stock price, after the dip it took recently due to backlash from their earnings call goofiness.
This $3B is a practice round. The $50M or so in wasted interest expense is the cost of an option to do a huge acquisition.