Why 2/3 Letters.com have More Value ..??

The main reason is because they are rare, short and only so few available. There's only 100 possibilities of 2 number domains and only 1000 possibilities of 3 number domains. Same for 2 letter domains, only 676 possibilities, and 3 letters, only 17,576 possibilities.

you type them faster

but they are actually meaningless..Most of doesn't find any end user.

They can be used for corporate acronyms eg HP, IBM... this is why.