3 g's for 20 hours is pretty harsh on the human body.
And keep in mind that power = thrust * exhaust velocity. For a constant power level, you lose exhaust velocity as you ramp up thrust, so if you can and do cram enough propellant through your fusion drive to ramp your thrust up to three g's, your exhaust velocity is likely to be much lower than 0.06 c.
Or, if you ramp up your power levels to keep your ISP constant as you increase thrust, you find that you pretty soon have a ship with (much) more engine power than the entire electrical generating capacity of Earth. Assuming that your 5000 ton drive makes up 10 percent of the mass of your ship (for a total mass of 50000 tons), that you have an acceleration of 3 g's, and that your exhaust velocity is 0.06 c, your thrust power is 25000 terawatts. By comparison, Earth's generating capacity is 15 terawatts. And only part of your engine power goes into your thrust. If the engine is 50% efficient (which is pretty good) it will put as much power into waste heat as it will into thrust.
Assuming (optimistically) that the engine doesn't vaporize itself, it will be as bright as the sun at a distance of 740 miles. Try parking the thing in LEO, and you will make the locals *very* angry. At a distance of 0.86 AU it will be as bright as Sirius. At the distance of Jupiter, it will still be as bright as a star of apparent magnitude 2.25 (for example, the constellation Cassiopeia is the one that looks like a "W." The two stars that make up the left-most arm of the "W" are about magnitude 2.25). On the other hand, most of that brightness would be in the form of X-rays or gamma rays, so you wouldn't actually see it to be quite that bright. You would, however, get pretty bad radiation burns if you were anywhere near it.
If you manage higher efficiencies (say 99.9999%, which would probably be pretty near impossible), you still have to worry about the fact that your exhaust stream is a weapon. Be careful where you point the back end of your ship.
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