There's a difference between keeping your magnesium levels at optimum levels and adding part 3B to balance Dr Holmes-Farley's formula.
The Part 3 mixture is specifically formulated to balance the Part 1 and 2 formulas by eliminating the buildup of sulfate over time (i.e., when you use Epsom salts--magnesium sulfate--without magnesium chloride).
The point I was trying to make earlier was that when using the Holmes-Farley supplement it would be best, if you can find it, to use 3B (mag chloride and mag sulfate) to balance the chemical equation.
Seachem says its magnesium supplement includes chlorides and sulfates. The issue is that it probably isn't formulated in the same ratio as Part 3B, so while it will increase your magnesium levels to desirable levels, it won't balance out the Holmes-Farley system, which should be your goal if using that system.
From what I hear, even if you follow the Holmes-Farley dosing exactly, you may still have to supplement additional magnesium if testing indicates it's necessary.
Of course, you can use 3B or any other mag supplement any time your mag level is low.