You decided to take the plunge then
I've looked at the manual on Eheim's website, so......
In another thread you say at the moment you have 2 x coarse blue sponges; 2 x fine blue sponges; 1 x green sponge; box cirax. I'll ignore the white pad as it has to be changed too often to grow that many bacteria.
According to Eheim, the pro 3 has eheim mechpro in the bottom basket; substrat pro in the next basket; white fleece; screened cover.
The mechpro is their mechanical filter, and looks like ridged black plastic 'noodles' according to Eheim's website.
Substrat pro is their biological medium, it's small rough balls which are actually sintered glass. I have this in my biopower internal.
The white fleece is filter wool to remove the small bits.
I have no idea if the screened cover is anything other than a plastic cover.
You can afford to lose up to a third of the Juwel media, no more. If necessary, lose the green sponge and one of the coarse blues.
What I personally would do is open the cirax box so you can get the bits out.
Then:
Put a coarse blue sponge in the bottom layer and surround it with mechpro. If you can fit both coarse blue in with room left over for the mechpro, even better. Chop the sponges up if necessary.
Put a fine blue, two if you can manage it, in the second layer, empty the cirax over them then top up with substrat pro.
You need to try and get three blue sponges in if you can't fit all four. If necesary, put half a fine one in each layer.
Run the filter like this for at least a month, preferably six weeks. Then remove the Juwel media a bit at a time with a month between each bit in this order:
Coarse blue, and top up with mechprosecond coarse blue if you squeezed it in, top up with mechprofine blue, top up with substrat pro
second fine blue, top up with substrat pro
coarse blue and top up with mechpro
second coarse blue if you squeezed it in, top up with mechpro
You'll never get the cirax out though, just leave it there.
During the swap over, keep the media wet all the time. If necessary, work in a bucket of tank water.
And don't get any more fish for a month. If you get more fish, delay the replacement of the old media a bit to allow more bacteria to grow for the new fish.
If I haven't already mentioned it in another thread, I can tell you how I got the Juwel filter box out of my tank with fish in it.