...SOME dryers have/had 3-prong plugs and needed 3-prong receps.  I just went through this about 2 months ago, in that we moved a dryer into my step-daughter's trailer that had a 3-prong pigtail, but there was a 4-prong recep, so we had to change the pigtail on the dryer.
  I lost my last 5/16" 1/4 socket down the back of the dryer.  GRRRR!Lyle
Just a note about just changing a pigtail on a dryer to fit the plug.
You can't do that on the newer ones. Well let's say you CAN but you should NOT.
On some of the newer ones, you 'have' to pull wires and install the correct plug. Long story short is just changing out the pigtail to fit an old plug will leave the newer dryer itself 'as the ground'!
I understand the theory of a 3 prong plug. My question is a 3 prong plug that doesn't have a neutral.
Take the time to read 'all' of this website that was posted earlier in this thread.. 
Here is the link to the '30 amp page' with all the dif plugs and wiring diagrams of the website. or click on 30 amp which is on the left side.
http://www.myrv.us/electric/