On My Edge III. Now, I know the Edge III gets a lot of abuse, and I'm aware that it isn't be best bridge, but I have a problem which I'm sure can be solved.

I don't use the trem, but I noticed it wasn't perfectly tuned. So I tuned it using a chromatic tuner (Boss TU2) and the whole guitar was hovering somewhere between E and Eb. I tuned it up to standard, and about a day later, it was in the same position as last time.

I don't know if this matters, but I use 9-42 gauge strings, and I have 3 springs at the back. Its been about 6 weeks since the strings were changed.

ANY help is much appreciated.

By the way, I have been thinking of blocking up the trem, but I don't know if that will solves the problem.
It could well not be a problem with the bridge at all. As you know the Edge III is a poor bridge, but I'd wager it's a more basic problem.

99% of the time when a guitar has tuning issues, it's simply because the person who restrung it, did so poorly. Have you stretched the strings properly? Did you wrap them around/through the tuning pegs correctly? Did you cut them to a better length? And so on and so on.

Even a $40 guitar with the crappest bridge, tuners and nut known to man, can hold tune so long as it's re-strung correctly.
Its probably not set up properly, in the trem back. Twist the screws inwards, and retune, give it a good bit of abuse, check again, keep doing it until it stays in tune.