kalyani,
There is nothing wrong with browsers and installing them can never slow your ssytem. But if any antivirus software is installed in your system it may slow the process of opening the browser and result is browser opens very slowy..They dont occupy much space on your RAM when compared to Anti virus softwares and OS
In general having different browsers is advantages than single one. For example consider Firefox which is best for opening sites like Boddunan.com which previously didnt work well in IE. Some features in Article section didnt work well. So if the site has specified any particular browser it is better to have it..
Google Chrome is best for high traffic sites like Facebook and games in facebook like Farmvilla.
I have three browsers installed on my system and i havent faced any clash problem or slow down etc...
Hope this information helped you.
Regards
Rajani