Adding on to #3, I would have left the stem alone as well...
#1 and #2 are underexposed... The colored background in #3 is done very sloppily. I'm not really a fan of any of them.
To help with #1, when you're shooting the white puffy dandelions, look for the fullest ones in the area. Don't shoot the ones that look like this and look like they are withering and/or dying.
With #2, just bump the exposure up some. Underexposed flowers are not pretty IMO. (Dammit, there I go with my opinions again... :roll: )
With #3, how did you make your selection? What program do you use for your editing? You more or less just need to make a cleaner selection of the dandelion and then it won't look so bad.