AirPort Extreme to Linksys fibre router

WiFi speed from the same extreme is around 120mb max.

That sounds like a 2.4 GHz 802.11n signal, which will max out at around 130 Mbps on an AirPort.

2.4 GHz is a stronger signal than 5 GHz, so your Mac or whatever you are using to test may be favoring the stronger signal.....not the faster....but weaker.....5 GHz signal on a dual band network.

Have you powered off the second and third AirPort and then assigned a separate name to the 5 GHz network on the first AirPort and then logged on to that specific network and checked speeds that way?

There are 3 airport extremes, connected in series via Ethernet from the Linksys router.

So, the first AirPort Extreme is connected to the Linksys router, the second AirPort is connected to the first AirPort and the third AirPort is connected to the second AirPort.....a daisy chain, correct?

What speeds were you attaining on the AirPorts before you moved to fibre?

Have you already tried resetting the AirPorts back to factory default settings and then setting them up again?

As we work through this, we will only be working with the first AirPort. The second and third will remain powered off until the first is working well. Then....we will work the second alone, and finally the third.

Are all of the AirPort Extremes the same model?

What is the model number? Look on the bottom of the device, it's usually embedded in the foam pad base.....starts with an "A" followed by 4 numbers.

Post back the number and we'll check to see what the max performance of the AirPort might be.

A1354 is capable of about 500 Mbps speeds at 5 GHz

5Th Gen....about the same