The past 3 or 4 times I have had my oil changed, they have had to fill the clutch fluid as it was a little low, like 3/4 of Max. What causes this, and is it normal as I don't remember this having to be done before?

A leak...

Check the clutch master cylinder from the inside of the cabin... there may be fluid where the rod from the clutch pedal is.

Or check the clutch slave cylinder near the transmission...

I didn't see anything on ther insdie of the cabin, but I have been told that there is a transmission leak, although the transmission fluid they never comment as being low, so could it be the clutch?

It's possible the clutch slave cylinder leaking. The clutch hydraulic fluid (brake fluid) is usually cleaner than the manual tranny fluid if it leaks...

You should check the tranny oil level too just in case it really is losing oil... you don't want it running low either...

Maybe it's a good time to change to Redline while you're at it.

Doesn't the fluid also go down as the clutch is worn out?
That is what I thought... I mean only a little at a time but...

That's true for the brakes where the pistons are extended due to the wear of the pads...

The throw out bearing is usually returned to it's default position by the clutch diaphram...

It is not normal. There is a leak. This kind of leak tends to get worse suddenly, resulting in catastrophic failure. Find it and fix it before it gets worse.

Clutch fluid only runs through 3 items - master cylnder, slave cylnder, and the hose. I had a leak in my system. It was from the hose to the slave cylinder. It costs about around 30-40 dollars from the dealer. It was a special order item so it may not be a common probelm. It is right near the starter. So if you have replaced your starter before, you should have no problem changing the hose. I also heard of someone else on this board having that leak also.