Should you buy Royal Dutch Shell plc, BHP Billiton plc and RSA Insurance Group plc before it's too late?

Long-term investors are better off forgetting attempts at short-term timing, because it's a fiendishly difficult thing to get right. On the other hand, selecting shares when they look undervalued is a key part of successful investing, but it's important to buy them when they're cheap and not wait too long.

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Don't miss the oil recovery

With a barrel of oil now back above $50, the highest it's been in more than six months, time could be running out to buy up our big oil companies while they're cheap.

From their low on 20 January, shares in Royal Dutch Shell(LSE: RDSB) have already regained 42% to today's 1,791p, so is it already too late to pick up a bargain? No, Shell shares are still down 36% from the pre-slump peak in 2014, and forecasts are already being revised upwards, with two years of predicted EPS growth giving us a P/E of 13.6 by the end of 2017. The dividend is expected to be maintained at a yield of 7.6% too, and though it won't be covered by earnings this year, it should be by 2017.

All of this is at today's oil prices, and the next six-to-12 months should surely see significantly more progress on that front. I'd be surprised if a barrel isn't fetching $75 by this time next year, possibly a lot sooner. It's really no wonder the City has a Strong Buy consensus out on Shell shares, but they won't be this cheap for ever.

Time for miners?

Prices of metals and minerals haven't picked up so strongly -- iron ore is up a bit, copper is still down. But anticipation is clearly building, and mining sector share prices are picking up. In fact, since 20 January, BHP Billiton(LSE: BLT) shares have climbed by 55%, to 876p, and that's despite the costs the company is likely to face over its part in the Fundão tailings dam failure at its co-operated Samarco iron ore operation in Brazil.

An upbeat operational review in April, in which chief executive Andrew Mackenzie said the company's recent structural moves will reduce output this year but should lead to stronger margins and returns longer term, has also helped. On fundamentals it's hard to value BHP shares at this point, with forecasts suggesting a P/E of a hefty 32 for the year to June 2017. But we might just have seen the turnaround point.

Financial rebound

My third turnaround pick is the financial sector, and today I'm looking at RSA Insurance(LSE: RSA). Our banks and insurance firms still look undervalued to me, some seriously so, as sentiment following the financial crisis hasn't yet recovered. And it's probably still being damaged by fears of the devastation that could hit the sector should we foolishly leave the EU.

RSA shares have picked up by 31% since their recent low on 9 February, but I say we're still a long way from being too late to pick up a bargain. In the two years since ex-RBS boss Stephen Hester took control, the company's restructuring has been bearing fruit, leading to an upbeat Q1 update in May which reported operating profits as "strongly up".

EPS growth forecasts for this year and next suggest P/E multiples of 14.8 dropping to 12, and a respectable 3.9% dividend yield by 2017. Are the shares cheap? They surely are.

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Alan Oscroft has no position in any shares mentioned. The Motley Fool UK has recommended Royal Dutch Shell. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.