The process is simple: 1) Type /macro, and choose an icon and name for your new hot button. 2) Copy the code below and paste it into the macro text box. 3) Replace the 3 drains on your action bars with this one macro ability.
Here's the code:
/cast [nomodifier,nochanneling:Drain Soul(Rank 1)] Drain Soul(Rank 1)
/cast [modifier:ctrl,nochanneling:Drain Mana] Drain Mana
/cast [modifier:alt,nochanneling:Drain Life] Drain Life
Now you might be asking yourself, "How the heck do I choose between three drains if I'm only using one button?" Well, if you take a look at the last two lines of code, you'll notice two modifiers: ctrl, and alt. Pressing one of those two buttons while pressing the macro hot button will dictate which drain is used. Pressing the macro + alt will cast Drain Life, the macro + ctrl will cast Drain Mana, and just the macro will cast Drain Soul (Rank 1).
And hey, if you don't like them in that order or want to use different modifying buttons, just adjust the code as necessary.
So there you have it. Three drains, one button, two spaces saved. I hope this comes in handy for ya. ;)