The percentage stated was an example - it varies based on the price of the house itself. Realtors will take a larger percentage on cheaper homes, and a smaller one on more expensive homes (and as mentioned before, it can be negotiable, unless the realtor works for an agency).
When I talked to some agents about selling/buying a larger property I actually had them tell me..."If it's over $(put in number here) then the commission % goes UP!" So now your 3% for a 500k home can become 5% for a 1,000,000 home.
Justification = Because it's a more expensive home/property/land/whatever the agent needs to work harder so their commission should go up accordingly.
Don't know if that's the case or not, because I can't afford homes in that bracket.