Well, in the first place, you can't guarantee that you'll be able to maintain rankings. This is because rankings depend on many factors totally outside your control -- most notably, search engine algorithm updates, and optimization efforts undertaken by your competitors.
In general, if you make sure your site is user-friendly with intuitive navigation and easy to use page layouts; fill that user-friendly site with excellent, unique content; and properly market the site to people in your target audience, then you'll maximize your chances of doing well in the search engines.
Of course, you may still face an uphill battle if one of your competitors decides to invest heavily in a site upgrade and optimization campaign. So, unless you have the ability to drive all your competition out of business before then, your best option is to continuously improve your own site to try to keep one step ahead of them. If you can make and keep your own site more informative, easier to use and better marketed than theirs, then they'll
be the ones facing the uphill battle.
In this case, since you're targeting the global ".com" as well as the countries of India and Australia, you're going to want to build relationships and promote your site especially to related businesses and potential customers in those two countries (for the .co.in and .co.au sites) and worldwide (for the .com site). It's not going to be easy to target the entire world (.com) as well as focusing on two specific countries, but if you have the right team(s) in place, it can be done.