please point out the mistakes in my statement

Can anyone please tell me which of the following statement is more appropriate. are there any mistakes.

I have been working for XXX company as a software engineer since january 2012, focusing on embedded software development

or

I'm currently working as a software engineer for XXX company for the past 3 months, mainly focusing on embedded software development.

or

I'm currently working as a software engineer for XXX company since january 2012, focusing on software development.

I'm trying to convey that I'm currently working in this company for the past 3 months and I'm mainly focused on embedded software developmet.

Is there a mistake in the following statement.

"I am writing to apply for the post of <software engineer> as advertised on the official website of XXX company."

Thanks in Advance.