How to wire up relays in a 3 wire lock?

Same connections. Just ground pin 30 instead of powering with 12 volts. It will then supply ground to the locks.

Just make sure pin 85 or 86 (whichever you choose) is still getting 12 volts.

No.  Nothing is needed on 87a, unless you were doing a 5wire lock.

For future reference, the way a standard SPDT relay works is;

85 & 86 - Coil - When this is energized (typically 12v, but other relay coils certainly exsist), the relay will close.

87a - Normally closed contact.  When the coil is NOT energized, 87a is coupled to 30.

30 - Coupled to 87 or 87a, depending upon if the coil is powered or not. Typically 30 is your load and 87\87a is your supply, though there are appliations in which 30 would be your supply and 87 and\or 87a would be your load.

HTH

87 - Normally open.  When the coil IS energized, 87 is coupled to 30.