I did a quick "Sram i-Motion 3" google search and it's definitely not that.

I did manage to do something though! I muscled the rubber cap and it popped open. I put lots of oil inside the hub, exactly how you'd fill your car up with petrol. I filled it up until some oil spurted out the other side. Since then, it's been leaking oil like crazy. Is this normal or is it broken?

I thought this had solved the problem. My bike rides a lot smoother, it's practically silent. Apart from the squeak!

I was sure it was the 3 speed hub, but now i'm not so sure. It sounds like it's coming from behind me, so the possible culprits are the 3 speed hub, the pedals or the rear brakes.

Here's what i've deduced along with the logic:
1). It's not the pedals. When I stop pedalling, but continue to move along (freewheeling) the squeak remains.

2). It's not the rear brakes. I took the rear brakes off and cycled along, but it was still squeaking.

3). It could be the 3 speed hub. It may not be the interior mechanism, but it could be the cog outside. This cog has a lot of oil on it though, so it might not be that.

I have been thinking it could be the rear mudguard. The tyre doesn't appear to catch on it.

Also, when i'm off the bike and spin the pedals really quickly, it doesn't squeak. It only squeaks when I get on the bike.

EDIT: I FOUND THE PROBLEM!

It was the wire holding the rear mudguard on the bike. The tyre was rubbing against this but, oddly, only when someone was sitting on it.

On the plus side, at least I figured out how to put oil in the hub. I also learnt a lot about bikes trying to find this one (rather trivial) problem.