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I have a Hero Sprint Axis (with 18T freewheel).I ride mostly on flat roads, but want more top speed and a lower gear for starting from standstill. Is it possible to just install a triple chainring with no modifications to the rear freewheel?

Can i get this triple chainring online in India? (or any dealer in Kharagpur?)

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    Way back, folks used to stop to change sprockets. Presumably you could get along with a derailer on the rear, just to keep the chain taut, and then change rings by hand (keeping some rubber gloves handy). But it's a little iffy, since the chain might tend to jump off the ring without a front derailer.
    – Daniel R Hicks
    Commented Oct 31, 2015 at 20:35
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    @DanielRHicks nice, but it's not what OP wants. He need the tipple to get "lower gear for starting from stand still" so he wants to change the gears on the fly while riding the bike.
    – Alexander
    Commented Oct 31, 2015 at 21:00
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    I do vaguely recall that rear hubs with 2/3/4 cogs, on a fairly narrow axle, used to be available back in the 70s. Probably not likely that they're still around, though.
    – Daniel R Hicks
    Commented Oct 31, 2015 at 21:05

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On that particular bicycle you'll have to remove the chainguard. Depending on the configuration of that bicycle's drivetrain, you might have to swap crankset parts. Worst case, both crank arms, the chainring (for a triple), a new crank spider (if it's a separate piece), and the bottom bracket will need to be replaced.

It is possible that chainrings could be added on. It's difficult to tell without being able to see it in person.

Once you have that, you'll want to find something to act as a chain tensioner. There are purpose-built chain tensioners, but if one of those is not available, you can use a derailleur. Make sure whatever you're using can take up all the slack you'll have when you're shifted into your smallest chainring.

You'll also want to ensure that your middle chainring forms a straight chainline to your freewheel cog.

Another cyclist put together a 2x1 project with his Surly Karate Monkey here.

I don't know about the availability of parts in India, but if you find it's not feasible, you might consider finding a bike with a 3-speed internally-geared hub. It takes the chain tensioner out of the equation, requires no front derailleur, and comes stock on lots of bikes. Hub gearing isn't as cheap as derailleur gearing, but it's less susceptible to damage and corrosion and requires less maintenance over time.

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Do you want to be able to shift between the chainrings? It kind of sounds like you do. To do that you need slack in the chain so that the front derailer can move it between the rings, and you need a way to take up that slack as the size of the front chainring changes. That is one of the roles of the rear derailer.

So assuming that you don't already have a rear derailer or chain tensioner you'd need to add one of those at the very least. Other than that (and possibly adding a front derailer) I can't see why it wouldn't work.

I'm sorry, but I don't know where you can get the triple in India. You may need to replace the crank arms as well to be able to mount all three rings.

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    One would likely need to replace the entire bottom bracket, to get a longer shaft. And certainly the right crank arm needs to be replaced to get one that accepts rings.
    – Daniel R Hicks
    Commented Oct 31, 2015 at 21:03

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