We are a newly started firm, works on embedded and PC based software development.
We have no manpower or resourses to concenttrate on many platforms.
Please suggest which will be ideal for a small scale company like us in the long term persepective.
For what purposes? You want to offer web design services or such? Or just running your own internet web site? Low budget rollouts for small places? High budget complex scalable n-tier enterprise web applications?
PHP: not my favorite, but there's a lot of resources out there. Hosting is dirt cheap, and it runs on basically everything.
ASP classic: this one is a non-starter... The NT4 days are over
If you know this stuff, you can make some $ doing maintenance work on it, or porting it to ASP.Net and such.
ASP.Net: it's pretty good imo, but the costs are higher (windows licenses to run IIS, lots of times the sites use MS SQL as a backend -- more $, etc). So hosting for it is MUCH more expensive. There are tons of great resources here too (MSDN, all kinds of stuff like the enterprise library, nice technologies like WCF, several very good/solid languages like C#, etc) and very good developer tools like visual studio (even the free express editions aren't bad -- visual web developer for web stuff)
But then again, there are more languages available too (Python, Perl, Ruby, etc etc). And language alone is only a small part of the whole solution. You will likely have to pick (or write) some libraries/frameworks/components and whatever else to do lots of the work (writing everything from scratch for every project isn't a realistic expectation, reuse is key). Even using some kind of CMS as a base for some sites is an option sometimes...
Sorry, I wish there was an easy answer to this, but there isn't really one.