Don't neglect your investments just because you've retired.

Some people believe that once you've hit retirement, your days of investing are over. While that could be true if you can live happily on Social Security and whatever defined benefit plan you may have, there are millions of American retirees who have nest eggs to consider.

If you combine withdrawals from that nest egg with the effects of inflation, your cash might not last as long as you think. And with today's elderly living longer than any others in modern history, it's important to take your retirement investing seriously.

Below, I'm suggesting three very broad ETFs from low-cost provider Vanguard Group to keep you invested, with a quick explanation for why I chose them.

So that you can benefit from long-term innovation trends
Generally, the idea is that you want less risky holdings once you hit retirement. Hopefully, you've built up a big enough nest egg that you aren't counting on your stock-picking prowess to provide for you in your 70s, 80s, and 90s.

But you shouldn't ignore stocks altogether. That's because, over the long run, nothing has been a better investment than the stock market -- returning over 6% per year after accounting for inflation. And if your retirement could last as long as 30 -- even 40 -- years, you want to have at least some of your money invested in it.

That's why I suggest the Vanguard Total Stock Market ETF (NYSEMKT:VTI). By buying shares of this ETF, you're getting exposure to almost everything -- over 3,700 different stocks, spanning all industries, and companies of all sizes. You'll own a piece of the largest energy companies, and some of the smallest biotech players.

And the best part is that the expense ratio for your investment is just 0.05%, or lower than 95% of the other ETFs out there with similar compositions. Because of its higher risk profile, I wouldn't suggest investing more than 25% of your nest egg in such an ETF.

So that you get regular dividend payouts
Dividends form the backbone of many healthy retirement portfolios. When reinvested over time, the compounding nature of such regular payouts can create a stable and growing pot of cash, while not exposing it to the same type of volatility that the Total Stock Market ETF will.

That's because dividend payers tend to be very stable businesses -- think power and energy companies, telecom giants, and those with sustainable moats surrounding their lines of business. That is, after all, how these companies can afford to offer dividends at all -- they have lots of cash left over at the end of every year.

But I'm focusing on a very specific type of dividend payer: those that have a history of growing their dividends over time. The Vanguard Dividend Appreciation ETF (NYSEMKT:VIG) fits this bill. The fund is invested in 178 different companies that show this crucial trait. About two-thirds of these companies are focused on just three industries: consumer goods, consumer services, and industrials.

The ETF currently offers a healthy 2.4% dividend yield, and its expense ratio is a very-low 0.10%. As with the Total Market fund, because your money is being invested in stocks, there is still risk involved, and I wouldn't suggest investing any more than 35% of one's holdings in such an ETF.

So that you've got some very safe income
Finally, we have what is probably the backbone of most retirement portfolios: bonds. Safer by nature, they may not offer the same potential for growth over time -- but that's not a problem if you've done a good job saving throughout your career.

An investment in Vanguard's Total Bond Market ETF (NASDAQ:BND) seeks to offer broad exposure to domestic, investment-grade bonds (no junk bonds here!). U.S. Government bonds make up a full 64% of the fund's holdings, while none of the other bonds are below a Baa rating.

Since inception, an investment in this ETF has returned 4.6% per year. That's nothing to write home about, but with the safety that bonds provide, and the low expense ratio of just 0.07% per year, it's well worth making this type of ETF your biggest holding in retirement.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.