Alnico 2 vs. alnico 3 vs. alnico 5 Stratocaster guitar pickup magnets
Alnico means a pickup magnet composed of aluminum, nickel and cobalt. Take the first two letters of each and you end up with al-ni-co. I've stated this before but am stating it again to remind readers that alnico refers to materials and is not a company name.
In a general sense, the number next to alnico refers to magnet strength. The higher the number, the greater the magnetic pull.
More often than not, Strat players prefer the stronger alnico 5 pickup magnet, and I'm of the same opinion.
Why would anyone want the 2 or the 3 when there's the 5? Pickup voicing. With the DiMarzio Strat pickups in particular, the choice of the magnets with less pull in the Area 61 mentioned above, Area 58 and Area 67 make for a tone with reduced bass and midrange but a ton of crisp treble on top.
The DP117 on the other hand, while having a lot more "blare" to it with more pronounced midrange and bass, has significantly less treble...
...which may be exactly what you want, depending on your play style.
Vintage style '50s and '60s electric guitar tones have tons of treble on top, so the choice of alnico 2 in the vintage-voiced DiMarzio pickups totally makes sense. More modernized tones for guitar soloing and blaring rock chords that the DP117 provides also totally make sense.
The only thing that wouldn't make sense is mix-and-matching alnico 2 with alnico 5, because one pickup will always be overpowering the other either in output or treble, resulting in a tone that really won't sound that great.
As for the alnico 3 magnet, that's also found in certain Strats as well, even for Squier.
For example, the Squier Classic Vibe '50s Stratocaster does in fact have pickups with alnico 3 magnets in them, while the Squier Classic Vibe '60s Stratocaster has pickups with alnico 5 magnets. In other words, a Classic Vibe Squier with the 1-piece maple neck has the alnico 3 magnets in the pickups and the one with the rosewood fingerboard has the alnico 5 magnets.
Which should you use for your Stratocaster?
Alnico 5 is the go-to magnet when not using ceramic because it agrees with most players. But for something that suits '50s and '60s rock along with super-twangy country tones, the alnico 2 and 3 magnets suit very nicely there.
This is not to say that alnico 5 can't do '50s/'60s/country sounds because it totally can, but when you really want that "sparkle" on top, the alnico 2 and 3 magnets really make a difference - provided the pickup is voiced correctly. Squier does voice their CV Strats correctly, and DiMarzio's Area series pickups are also voiced very well for what they are.