I don't think U can get 50inch and above within 3k. Especially for LCD/LED. For LCD, I barely seen size bigger than 42inch but for LED, December last year the price for 46inch LED around 3.5-4K. Within your budget, U will easily get a 40inch LED and maybe if the price drop because of new year U can get a 46inch. Most of LED now offered a 3d but none 3d, I'm not so sure if it still available or not especially for above 50inch. For 40inch LED, U can get the price at below 2k.

Which one U can afford? LED is super expensive. If a money not a problem then it is up to you. But If you have limited budget, plasma is a good choice.

To answer this question, better you as yourself what you do often with your TV? Most of people tend to agree plasma is better for people who like to watch movie. The reason is about natural picture and bigger screen size.

I believe your TV can play through USB. Just check in your manual what format supported.