D-commissioning a bathroom won't achieve anything for the showers in the other two, I have never noticed a problem with simultaneous showers, two are original and have separate hot/cold feeds from the tank, the other is fed from a combination bath/shower tap.
Three bathrooms - why not, we have five bedrooms. Two are ensuites and I thought that in this modern age we all showered every day - try that with one bathroom and 10 people in the house. Never been more than 3 Yorkshiremen in the house to my knowledge, my son was born in Derbyshire and my brother-in-law in County Durham.
The tank in the loft is already raised and would probably only go up another foot which would have little effect (IIRC 1bar is about 30ft of water height), it is also over a meter in diameter and there is an enormous structure supporting it - full it probably weighs a ton or more. Taking it out would simplify the loft insulation and I could also move the heating header tank out of the way.
My preference is to change the tank but wanting to check other options, will get a couple of quotes and see which way we go.