I was a US Army officer commanding tank units until 2015. This is pay-walled so I can only see the first paragraph, but if the article doesn't say, the 3:1 rati

I was a US Army officer commanding tank units until 2015. This is pay-walled so I can only see the first paragraph, but if the article doesn't say, the 3:1 ratio to make an attack favorable is not meant to be a ratio of pure manpower count or weapons count. You're supposed to estimate the total firepower each side can actually bring to bear, so it should account for differences in training and readiness, equipment quality, intelligence quality, and anything else relevant (of course, your assessment of force ratio is already assuming you have high quality intelligence). As mentioned elsewhere, it's also a very rough starting point assuming open terrain. If the defender is defending extremely easily defensible terrain, such as cities you're not prepared to just outright level to kill them, you need to adjust the ratio upward as appropriate. As with anything, you're supposed to exercise some discretion and judgment as a commander, otherwise what the heck are they paying you for if the job could be done equally well by a spreadsheet?