I have a 12ft by 12 ft L/W & 10 ft high porch
i want to install a ceiling fan
there exists only 3 cross beams
and the ends and one across the middle
it is a 2x6 that sits on top of side beams

Can I attach directly to the 2x6? or how best it attach it
another 2x6 along side with some sort of block between them??

 

I've installed ceiling fans in rooms I've gutted by installing an adjustable brace with an electrical box especially designed for this purpose. What I'm getting at is this: I'm 200lbs and was able to hang from the brace and do pull ups after I installed it.

If you wouldn't even consider letting me do pull ups on your 2x6, then don't do it. A 2x6 that is totally secure and has room for you to run your wiring on top, would in my opinion be fine. Make sure the 2x6 doesn't twist or bow at all, you don't want a ceiling fan coming down on someone's head.

Not using an electrical box mounted to the 2x6 first is most likely against code. You should really check with your local building dept before going ahead with this. <----that's my disclaimer.