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I'm looking for answers that focus on changes in internal anatomy that allow humans to survive in a high-g environment. If dramatic visible change can be avoided, that'd be great.
Problems
Circulatory: Blood is much harder to pull away from the ground. Energy needs to be delivered to muscles at a higher rate
Muscular: Muscles are under more stress due to lifting heavier loads
Skeletal: The skeleton need to support a load with 3x the weight
The first thing to go would be posture. This whole standing upright would have to be reconsidered. After all, putting the most oxygen hungry organs in the body up at the top is asking a lot of our earth-born heart and veins. Completely prone would become everyone's preferred pose which might cause a sharp decline in personal productivity and mobility.
We would also have to give up salt. After all, who needs high blood pressure when your blood already ways three times as much as normal. Limiting dietary iron to induce anemia would become good medical advice for putting a little spring back in your day.
At least dieting would get easier. Loosing multiple pounds per week is easy when even the lightest broth feels like thick pea soup on the tongue. Pigging out on ten pounds of ice cream would still leave the gallon container half full.
Okay, so we aren't going to get very far on this planet in our current condition. Time for the genetic scientists to step in...
The name of the game is decentralization. Instead of relying on a single centralized pump, smaller fluid flow assistance organs should be spread out around the body. No major vein or artery should be run for more than a foot before reaching the next of these smaller hearts. In blood pathways which defy gravity, the hearts should be even closer together.
In later generations of the genetic manipulation, the entire idea of hearts and veins will be abandoned. Instead the blood passage ways would themselves be pumps. Every inch would be segmented with valves and lined with muscles such that no portion of the system simply holds blood. Every part contributes to the circulation of the body's blood supply.
Along with the enhanced plumbing, it would probably be wise to rearrange some of the systems. Eyes and ears don't weigh much so they can stay up at the top, but the brain could migrate downwards. Men have been accused for centuries that we keep our brains in our pants. Now in a very literal sense, that would be true of women as well.
We would want to keep the overall humanoid shape unchanged for aesthetics as well as to assist with dealing with other humans from other worlds. But we would not want to put anything heavy up in the head section with the ears and the eyes. So maybe just a cartilage shell containing helium. We are the airheads and proud of it!
Which leaves the bones and muscles... that is the tough part. It is sort of like building a muscle car. A bigger engine produces more power but it weighs more meaning it takes more power to move. The geneticists would have to work closely with material scientists. The age of all carbon and calcium would have to end. Light weight metals and silicon cording might replace the bones and tendons, allowing our current muscle mass or maybe even a little less to carry the load. If not, then we would have to plunder the animal kingdom for a muscle upgrade. Human muscle to strength ratios are abysmal compared to many animals...
