Thanks to exchange-traded funds (ETFs), which allow you to invest in multiple companies at once, you can cover a lot of ground with just a few investments. This cuts down on research time and reduces some of the risks that come with investing in individual companies.

If you're looking for three ETFs that can supercharge your retirement savings, look no further.

1. Vanguard Total International Stock ETF

Part of having a well-diversified stock portfolio is investing in companies outside the U.S. It can help with industry diversification and currency diversification, and ensure you're not always fully dependent on the U.S. economy. Researching U.S. companies can be time-consuming enough, but added layers to consider -- such as local economies and political stability -- can make investing in individual international companies a bit tougher.

That's what makes the Vanguard Total International Stock ETF (VXUS 0.87%) a good option. It contains over 7,900 companies of all sizes in both developed and emerging markets.

Developed markets are countries with higher levels of economic stability and infrastructure, such as Japan, the U.K., and Germany. Emerging markets are in the process of developing infrastructure and stable financial systems, like Brazil, India, and China. Investing in companies from both markets can give you access to some stability as well as high-growth opportunities.

Region-wise, the ETF is broken down as follows: Europe (40.1%), Pacific (26.7%), Emerging Markets (25.3%), North America (7.4%), and the Middle East (0.5%). Notable companies within the fund include Taiwan Semiconductor Manufacturing, Nestlé, and Samsung.

You don't need the bulk of your portfolio in international companies, but a good guideline is to aim to have around 20% of your stock portfolio in that category.

2. Vanguard Mid-Cap ETF

There is often a size-risk trade-off in stocks. Larger-cap companies are generally more stable because of the resources they have to withstand broader economic conditions, but this size tends to limit their growth potential. Conversely, smaller-cap companies are more prone to volatility, but their size gives them higher growth opportunities.

Mid-cap stocks (companies with a market cap between $2 billion and $10 billion) can be the sweet middle ground. They're small enough for lots of growth room, yet large enough to have the resources needed to weather many storms that may come their way.

With a 0.04% expense ratio, the Vanguard Mid-Cap ETF (VO 0.92%) is a low-cost option that covers a lot of ground. It contains over 340 stocks fitting into both the value and growth categories. The top five sectors in the ETF are: Industrials (15.7%), Consumer Discretionary (13.6%), Technology (13.5%), Financials (13.4%), and Health Care (10.7%).

3. iShares Core S&P 500 ETF

If there was one investment I had to rely on to take me to the retirement finish line, it would undoubtedly be the S&P 500. It tracks the 500 largest public U.S. companies, so an investment in the S&P 500 is essentially an investment in the U.S. economy.

The iShares Core S&P 500 ETF (IVV 0.64%) is a good go-to option because of its low cost (0.03% expense ratio), diversification, and abundance of blue chip stocks. The ETF is market cap-weighted, so larger companies account for more of the fund. That makes the S&P 500 more top-heavy than other options, but it's great companies like Apple and Microsoft leading the charge. 

Famed investor Warren Buffett is one of the biggest preachers of consistently investing in an S&P 500 ETF to satisfy investing for retirement. He says it "makes sense practically all the time," and looking at the S&P 500's historical performance, it's hard to argue with him.

Chart showing the S&P 500's total return rising overall since 2005.

Data by YCharts

The S&P 500 has historically averaged around 10% annual returns and outperformed many actively managed (and therefore more expensive) funds on Wall Street. Nobody can guarantee this will continue going forward, but there's a reason the S&P 500 is the benchmark for stock performance.

I would personally keep over half of my retirement stock portfolio in the S&P 500, increasing the percentage as retirement nears.