The Mk 3 is a pretty staightforward and sturdy gun internally. Most of it is pretty self-evident. The spring is under little pressure, I have done these without a compressor.
Here's a parts diagram:
If you can lay your hands on a copy of Hiller's "Collector's Guide to Air Rifles," it contains tons of drawings and pretty clear directions on how to take a Mk 3 apart, and re-assemble it. Also a pretty complete history of the gun. VERY highly recommended.
The rear of the receiver threads off similar to the older Weihrauch models. The only tricky bit is the lug the front trigger guard screws into (part 42 in the drawing). It must be removed before the rear section comes off, and it has REVERSE threads--turn it clockwise to UNSCREW it. After taking the rear section off and removing the cocking link, the spring and piston will come out.
The trigger and sear are separately pinned to the rear cap, and have a spring between them. To put back, pin the sear in first with the spring hole facing down, stick the spring in the trigger, and work it in. It isn't complicated but does take a certain amount of wiggling about to get it back in right. You don't have to remove the trigger to get the spring and piston out, though.
If you take the loading tap out (again not really necessary), there is a small spring and ball bearing under the plate on the left side, don't lose them. Best way to loosen the tap is undo the screw that holds the tap lever on a few turns, then tap it gently to pop the tap out of place. Then undo the screw the rest of the way and the tap and lever will be loose. If you try to pry the lever off you can accidentally break off the square stud it's mounted to, and then you will have an un-shootable gun.