I have a computer with such PSU, it even has a fourth connector for 5V standby and soft power-on, just like ATX. However, the motherboard hasn't got the connector soldered on so the 3.3V cable is not in use. You may be able to run your board without it aswell.

I have seen that connector mostly in early PCI based motherboards with Intel chipset, such as late 486 (Saturn, Aries) or early Pentiums. I never had one of those boards myself, however the manuals mention that it's a 3.3V power connector "to maintain strict compliance with the PCI standard". No clue what that means.

You could always just use a modern ATX supply and rewire it with the appropriate Molex connectors.

TheMobRules wrote:

I have seen that connector mostly in early PCI based motherboards with Intel chipset, such as late 486 (Saturn, Aries) or early Pentiums. I never had one of those boards myself, however the manuals mention that it's a 3.3V power connector "to maintain strict compliance with the PCI standard". No clue what that means.

When PCI was introduced, the AT PSUs back then had not enough juice on 3.3V to fulfill the PCI standard. That's why this connector exist. You don't need to care about, unless you plan to connect it to a pre-1994 AT PSU.

derSammler wrote:

When PCI was introduced, the AT PSUs back then had not enough juice on 3.3V to fulfill the PCI standard. That's why this connector exist. You don't need to care about, unless you plan to connect it to a pre-1994 AT PSU.

But standard AT power supplies do not provide 3.3V, just 5, 12, -5 and -12 volts. The 3.3V line was introduced by the ATX standard...

Board will run without 3.3V connected without any issues, all mine do at least.
Also, there are ATX->AT adapters on the ebay that have this third 3.3V connector, but be careful if you buy one of those, they are often wired with reverse polarity!. Please make sure to check there the ground is on your board and on this adapter before plugging it in.

Acer liked to do that. and yah ^^^^ double check that, Acer also had funky wired "AT" connectors.

TheMobRules wrote:

But standard AT power supplies do not provide 3.3V, just 5, 12, -5 and -12 volts. The 3.3V line was introduced by the ATX standard...

The point was that this connector was only added to get PCI approval. When not connecting, 3.3v is generated from 5v (or not provided at all), which however does not fulfill the PCI standard, since the sum of current is too low (6A is required on 3.3v alone to comply PCI 2.1).