B737-700 CREW HAD RWY INCURSION ON TAXI OUT AT SAT.

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I WAS MAKING THE VERY SHORT TAXI FROM GATE TO RWY 30L IN SAT. I WAS PUSHING TO GET THE CHKLIST FINISHED WHEN GND TOLD US 'CLRED ONTO RWY 3 FOR FULL LENGTH.' I WAS LOOKING AND RESPONDING TO THE CHKLIST AND DID NOT NOTICE WE WERE STILL ON GND CTL. WHAT I THOUGHT I HEARD WAS 'CLRED ONTO RWY 30L FOR FULL LENGTH.' THERE WAS AN ACFT SITTING ON RWY 30L JUST PWRING UP FOR TKOF AND I WRONGLY ASSUMED I WAS TO DO THE SAME THING. THERE WAS NOT A 'HOLD SHORT OF RWY 30L.' I SIMPLY HEARD THE 'CLRED ONTO RWY' PART AND MISTOOK RWY 3 FOR RWY 30L. MY BAD. THE FO WAS HEADS DOWN READING THE CHKLIST AND HAD NO IDEA WE WERE SO CLOSE. AS SOON AS HE LOOKED UP HE YELLED; BUT IT WAS TOO LATE. ABOUT 20 FT OF MY JET WAS ON THE RWY. THERE WAS A GA ACFT ON FINAL. LOOKED LIKE A GRUMMAN TRAVELER. THE GRUMMAN WAS KIND OF HIGH SO I DIDN'T SEE HIM FROM MY SIDE OF THE COCKPIT. HE WAS TOLD TO GO AROUND AT FIRST; BUT THEN CLRED TO LAND LONG. THE GRUMMAN PASSED WELL ABOVE US AND LANDED AROUND THE LAST 1/2 OF THE RWY. HAD I NOTICED WE WERE STILL ON GND CTL I WOULD NOT HAVE TAXIED ONTO THE ACTIVE RWY. HAD I NOT BEEN PUSHING THROUGH THE CHKLIST I WOULD NOT HAVE TAXIED ONTO THE ACTIVE RWY. HAD I NOT HEARD THE 'CLRED ONTO' OR HEARD 'HOLD SHORT' I WOULD NOT HAVE TAXIED ONTO THE ACTIVE RWY.