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Successful investing is as much about avoiding potential losers as it is picking winners.

Okay, so we know that the occasional big winner can more than offset the impact of a number of smaller losers. But when it comes to many investors – particularly those approaching retirement age who tend to be more conservative with their wealth – they aren't looking for big winners. They're looking for businesses that can grow their wealth comfortably, over time. Hence, the gains on many of these businesses may not be enough to offset a number of losers, forcing the investor to succumb to below-par returns.

With that in mind, here are three shares I am avoiding in 2018, at least based on their current share prices. Although they could go higher from here, the risks do appear to outweigh the potential rewards, in my opinion.

Commonwealth Bank of Australia ASX: CBA is a favourite among investors. For one, it is very familiar to most Australians while it also has a solid long-term track record and pays a handsome, fully franked dividend. But the bank's shares aren't cheap: at this price, a lot of things would have to go right to see the shares continue to rise over the coming years, in my opinion, leaving plenty of downside risk for investors who buy today.

For one, it is extremely reliant on Australia's housing market: if cracks do begin to appear in that market, the banks could be very vulnerable. Indeed, as providers of capital the banks are also vulnerable to a potential pullback in the Australian or global economies in general. I'm not pre-empting another crisis, but rough patches are something that investors should be prepared for at all times, regardless. This isn't to say I would rush out to sell the bank's shares, but I think it's one for investors to tread carefully around.

Mining heavyweight BHP Billiton Limited ASX: BHP is another blue-chip business that acts as a core holding for the portfolios of many investors. BHP had a great run in 2016 thanks mostly to soaring commodity prices (iron ore, coal and oil, in particular), and 2017 saw the shares rise another 10%. The problem is, it seems likely that further gains in BHP's share price will be largely dependent on those commodity prices strengthening further. That is something BHP has no control over, making it a somewhat risky play to build wealth in 2018.

Early shareholders of private health insurer Medibank Private Ltd ASX: MPL are still sitting pretty since the group's initial public offering (IPO) late in 2014. Retail investors got their shares for $2.00 in the float, compared to the current share price of $3.20. But the company itself doesn't appear to be performing quite as strongly: it's losing market share to competitors and isn't said to have very good customer service levels. Of course, there are things to like about the insurer, but I think there are better investment opportunities in 2018.

Here Are 3 Blue Chip Shares That Should Be On Your Radar in 2018

This company's dividend is eye-popping indeed. Its reliable cash flows support a high payout ratio, and the company's stash of franking credits are the cherry on the top of the dividend cake. Based on the last 12-months of dividends, shares are offering a fully-franked 7.8% yield, which grosses up to a whopping 11%, when those franking credits are included.

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