Hep-B is an acute, viral infection. It will cause severe fatigue, and simple supplements will not help. His infection needs to run its course, taking up to 6 months or so. Doctors will typically sit back and monitor.
The magnesium is taken with the Vitamin D3 to help regulate calcium. I don't know if he's able to take Vitamin K2 (some meds conflict...like blood thinners), but the Vitamin K2 directs calcium in the body and stops it from being deposited into the arteries.
Also need to stay hydrated to flush excess calcium out of the body.
Blood work
25 OH Vitamin D
It shows D3 levels.
30-100ng/mL is lab normal range
Anything below 50 isn't good
Optimal health is 60-80ng/mL
3,000iu D3 daily is not excessive.
400iu is a dose for a baby. 800iu barely prevents rickets.
I take 10,000iu daily and raised D level from 19.2 to 61 in 11 weeks.
He needs magnesium to take with the daily D3.
Insofar as taking vitamin D3 supplements, my feeling is that you should have an inexpensive D3 level test performed before taking supplements. A so-called "normal" level is 30 on the test. 3000 IU per day is not excessively high. Patients are often given 50,000 IU in a once weekly supplement. Excess vitamin D3 can cause alterations in blood calcium, although that would be unlikely at the dose you have recommended. My guess is that the tiredness is related to the hepatitis. He needs labs and a full in-person work-up by a physician.
Your brother's tiredness may be due to his hepatitis and/or the medications he is taking for that. However, 3000 IU of vitamin D3 per day sounds much too high, so that could well be a factor, also depending upon the reliability of the brand's ingredients. 400 IU is a good starting point.