triple chainring with with single speed freewheel 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?)
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.