New build, new bee. I know that I will buy two raptors in raid0 and one WD 750 for storage. The dumb question, most of the cases have only two slots that are 3.5 Because this is my first build, do I need 3 slots that are size 3.5? Bare with me, before I reseach it, is my Sata dvd burner a 5.25? An external 5.25 means you can access it like openning your cd drive? Are all internal HDs 3.5? Thanks for your patience.

Generally speaking you won't need an externally (that's the key word... externally) accessible slot to mount a HD. Most decent cases have the ability to mount more than 3 HDs. What cases are you looking at? I'm pretty sure my Thermaltake Armor will hold 8+ drives... although you can't quote me on that. Keep in mind if you're in a real crunch for 3.5 bays, it should be easy to find a 5.25 to 3.5 adapter/kit to let your HDs sit in the same spots as your optical drives (you'd just leave the cover plate on so it didn't look retarded)

I would say that "most" [mid tower] cases offer four or more internal 3.5" bays. Perhaps you're seeing two for the number of external bays; these are for floppies (which are hardly needed any more) or memory card readers.
I'm not sure when I last saw an internal 5.25" bay. A DVD burner, SATA or IDE, would use an external 5.25" bay. A mid tower typically has 3-4 of those. Adapters can also be used in them to mount internal 3.5" drives.
All this changes if you are looking at a Micro-ATX case (a mini-tower or SFF case). I just built my new rig in an Apevia X-QPACK2 SFF case. I put two hard drives in it, and could have added a third (with an adapter in a 5.25" bay), along with a SATA DVD writer. It takes a standard PSU, but won't fit a top-performing tower CPU cooler, so I'm using the stock (a Zalman 7000Cu would fit).