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Intel's Socket 3, used for 486 processors, was a 19×19 pin grid array socket. However, all compatible processors, to my knowledge, used 17×17 PGA packages. What was the point of the extra pins around the perimeter of the socket? Were they even connected to the motherboard at all?

If the pins are actually used by something, what were their functions? All pinout charts I can find online only show the pins of a 17×17 PGA processor.

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The extra pins were forward-planning, on both Socket 2 and Socket 3. Most of the extra pins are used for power (Vcc) and ground (Vss), which is useful to provide more power to a CPU. The other pins are keys, a new INIT pin (F19), and signals used for enabling and controlling the write-back L1 cache. (See the socket 3 specifications in the 486 family datasheets and the Pentium OverDrive datasheets.)

The only CPUs to use the extra rows of pins are the Pentium OverDrives.

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    @mnem Socket 2 is 5V only. Socket 3 supports 5V and 3.3V (or 3.45V) operation. The processor has a voltage detection pin that is used by some boards to automatically switch the voltage. The Pentium OverDrive has its own regulator, so it does not require a board with a 3.3V regulator.
    – Michael Karcher
    Apr 28, 2020 at 18:33
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    Technically the 5x86 was supported in Sockets 1 and 2, with replacement VRMs.
    – Stephen Kitt
    Apr 28, 2020 at 18:41
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    @dim I imagine multi-layer PCBs help a lot there...
    – Stephen Kitt
    Apr 29, 2020 at 10:20
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    @dim I think the 486 layout largely assumes at least four layers, with a power plane connected to the Vcc pins and a ground plane connected to the Vss pins. Adding more Vcc and Vss pins on a through-hole component doesn’t increase the layering requirements, AFAIK.
    – Stephen Kitt
    Apr 29, 2020 at 12:09
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    @StephenKitt It's intel Document 290544, see for example at db.zmitac.aei.polsl.pl/Electronics_Firm_Docs/PENTIUMIII/pentium/…
    – Michael Karcher
    Apr 29, 2020 at 15:25
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Socket 3 did not have more pins than needed. It was designed to support the Pentium OverDrive CPUs that Intel released in the mid-1990s. These CPUs could have up to 237 pins, while the Socket 3 allowed 238 pins.

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    So it was more pins than needed at first, but it was future-proofing, and did actually get used by a socket 3 pentium, got it. I didn't know there were any socket 3 pentiums.
    – Hearth
    Apr 28, 2020 at 14:47
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    It wasn't so much future-proofing as that they already had the upgrade chip on the drawing board when the Socket 2/3 came out, so they were asking purchasers to future-proof for an upcoming product. That product just took longer than expected... long CPU delays aren't a recent thing.
    – SilverbackNet
    May 14, 2020 at 1:37

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