which is better with this three development boards. Intel Galileo, Raspberry Pie and Beaglebone black?
They each have strengths and weaknesses. Will have to provide some context to narrow the discussion. What do you plan to do with the board?
Asking which is best might work for generic commodity parts, but asking that for three totally different micro-controller development boards is simply madness. So we can only ask back, BEST FOR WHAT? Best price? Best for the range of project you have planned? And even then you would need to consider if any given responder has actual hands on experience with all three, or is he/she just winging it?