Re: Magnet Swapping Opinions
A3's are like an A2, in that they are low output, but they're brighter than an A2 (which is heavy mids & little treble).
I've used a number of replacement magnets, mainly A4's & A8's. They're good to warm a bridge HB or P-90 if an A5 is too bright.
I haven't used many A3's, but I see them as useful when you have an A2 neck PU that is too dark or muffled. Or as a substitute for an A5, when it's too loud or bassy in that slot.
I don't think A3's would work that well in a bridge PAF, as an A5 would give more output and cut. An A3 bridge PAF may also be overpowered by whatever's in the neck. A3's would be better-suited to the bridge slot of a high output HB, although they're certainly not wildly popular in those. I don't recall anyone using one in a Custom (C3?), but it may work for certain applications. The CC is too dark for some of us, and a (relatively) brighter magnet is often preferred, like an A8 or A4, which still retain some mids.
A3's also have uses in P-90's, when paired with another magnet. I have a Phat Cat in the neck of an LP with a A5/A3 magnet pair. Eliminates any boominess that twin A5's might have in a fat mahogany body. In an SG, that same magnet pair was bright & 'chimey' in the neck.
Of the commonly available alnicos (2, 3, 4, 5, 8), A3's get the least use. They're pretty specialized and players usually choose another magnet that does the job better. By the same token, A8's are also specialized (suitable only for the bridge), but are rapidly becoming popular because they 'correct' some of an A2's faults: low output, rounded high-end, and a loose low-end. A8's don't have quite the 'color' of an A2, so you make compromises no matter what magnet you use.
In an Antiquity Set, I'd consider an A3 or A5 for the neck if the stock A2 was too dark or muddy, but I'd never use an A3 in an Ant bridge.