I believe that I once read in a book about about mathematics that Galileo Galilei (1564-1642) popularized the square cube ratio in relation to biology.
As the dimensions of a shape change, the shape's area changes with the square of the dimensions while the shape's volume changes with the cube of the dimensions.
So a man 10 times as tall as your 6 footer would be 720 inches tall. The area of a horizontal slice through his leg bones would be 100 times the area of such a slice through a 72 inch man, so the leg bones would be 100 times as strong, but the man's volume and weight would be 1,000 times that of a 72 inch man, so each horizontal square inch of leg bone would be subject to 10 times as much weight as was the case for a 72 inch man and would probably break under the stress of standing, let alone walking or running.
A man 100 times as tall as a six foot man would be 7,200 inches tall and his bones would have a horizontal cross section 10,000 times that of a 72 inch man, while his volume and weight would be 1,000,000 times as great. Thus his leg bones would be subject to 100 times as much weight and pressure and would certainly snap.
And the same thing goes for the cross section of his muscles compared to his volume and weight. His muscles would be terribly under powered for the weight they had to move. That includes his lung muscles and his heart muscles.
So Galileo proved that interpreting fossil bones of megafauna or dinosaurs as the bones of human shaped giants tens or hundreds of feet tall was not reasonable and those bones should have belonged to highly nonhuman creatures with body plans capable of supporting their vast weight.
A three inch tall man would be 0.0416666 as tall as a 72 inch man. Thus his leg bones would have 0.0017361 times the cross section of a 72 inch man's, and his volume and weight would be 0.0000723 that of the 72 inch man. Thus his leg bones would bear only 0.041666 the weight per unit of area of cross section, and so would his muscles. He would have about 24 times as much strength for his weight that a 72 inch man would have.
There actually are animals with bipedal locomotion similar in size to your three inch man. They are kangaroo mice and similar tiny rodents.
And you will notice that their limbs are very slender in proportions, because they don't need thick limbs to move at their tiny size. I suggest that your tiny 3 inch man will have very slender limbs instead of thick human limbs. Watch a television show or movie with child characters and picture your 3 inch man with the skinny arms and legs of little boys and girls instead of the bulging biceps of a a bodybuilder, and then make his arms and legs much skinnier than that.
And there are a number of other adaptations that your 3 inch man would have due to his tiny size. So he would look a bit different from a normal 72 inch man shrunk by a mad scientist's shrink ray to three inches tall.