3 Gallons is right.  Propane tanks are only filled to about 80%, so 3/4 isn't too far off.

Bill
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    I've had my tank filled twice in recent weeks (had a propane leak) and the
 first time it took 8 gal., the next time only 3 gal. I thought the first time he
 didn't fill it all the way, but the second time -- geez. And the second time I
 told the guy he had to open the overflow valve all the way, not just crack it,
 but he said he didn't. Also said he could only fill it 3/4 full for safety
 reasons. What gives? Is this because of the new regs that made all those old
 20lb cylinders obsolete?