I'm assuming you're talking about a Mac PRO here, not a MacBOOK Pro, MM??? (No, you cannot stuff three drives into a MacBook Pro - even little solid state ones!!
)
Anyway - "eSATA" stands for
external SATA - it's a connector allowing for external SATA connections for external drives - like Firewire and USB allow for ATA and SATA drives to be connected externally (though both of those protocols require bridges - eSATA does not). It hasn't anything to do with the actual connection
on the drive itself - it will be SATA.
So, yes, absolutely you should be able to take those three SATA drives out of their eSATA enclosures and stick 'em in your new Mac Pro. Just unplug the SATA cable INSIDE the drive enclosure and plug the drive into the SATA connectors in your Mac Pro.
Which Mac Pro did you get? (so we can all be jealous...)