Traditionally the month of September has a reputation for being one of the worst months for financial markets. With the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) down around 4% already this month, it certainly is living up to this unfortunate reputation.

But it looks as though some shares didn’t get the memo this year. The following three shares have had a fantastic month and put on sizeable gains. Here’s why:

Class Ltd (ASX: CL1)

The share price of this fantastic self-managed super fund software provider has risen 18% so far in September. Investors appear to have been fighting to get their hands on its shares since it reported incredible full year results that revealed a 71% jump in net profit after tax and a 37% lift in billable portfolios. Furthermore, at the start of the month it was announced that Class would be added to the S&P/ASX 300 (Index: ^AXKO) (ASX: XKO) on Friday’s after market rebalance. This is likely to have brought the company onto the radar of fund managers up and down Australia that were previously restricted from investing in its shares.

Sigma Pharmaceutical Limited (ASX: SIP)

The company behind the Amcal and Chemist Warehouse brands has seen its shares jump around 12% so far this month. A large portion of these gains came at the start of the month when Sigma released half year results which showed a 17% increase in earnings before interest compared to the prior corresponding period. This was well ahead of the company’s prior guidance of 10% growth. The company’s launch of an online store in China was another highlight. Personally I feel its shares are looking a little expensive now and wouldn’t be too surprised to see a slight pullback occur.

Whitehaven Coal Ltd (ASX: WHC)

This coal miner’s shares have gone gangbusters in September and climbed a massive 21%. As you might expect a jump in the price of coal has been the biggest contributor to this increase. According to the Australian Financial Review the Chinese government has been limiting its coal mine production to just 276 days per year. This lower production has caused coal prices to spike over the last month, much to the delight of its shareholders. Unfortunately it is unclear how long coal prices will stay at these levels, so I would personally caution against an investment in the miner.

Finally, of the three shares above I would only recommend investing in Class at present. But before investing in it I would highly recommend taking a look to see if you own these three wealth destroying shares. Each could be harming your portfolio right now and might be best swapped out.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia owns shares of Class Limited. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.