Can I place a block of diamond and break it with a fortune pickaxe to get 2 or 3 blocks of diamond?
No. Fortune does not work on diamond blocks. Fortune works on:
All of these are blocks that drop an item, you will never get multiple blocks. For blocks like glowstone that can be recreated with glowstone dust, you do not get more than what is required to create the block.
No. Fortune works on ores only, and only ores that drop a different item, for this reason. Fortune is meant to get more diamonds from your ore, not to duplicate items.