In RMFRP, in the English version, under Static Maneuvers in the bulleted list of modifiers, it says:
* If a skill applies to a maneuver, add the skill bonus from the character's character record.
* If there is no specific skill, the GM may decide that three times an applicable stat bonus will modify the maneuver.
For Moving Maneuvers it says "If no skill applies to the maneuver, add three times the character's Ag stat bonus."
So, yes, 3x stat bonus is an official rule.