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The Viessmann design isn't new - it's a variant of the 'old' Range Powermax (sold to Potterton and then discontinued when they went back to a boiler+cylinder design under the same name for the 'gen-2' product around 2004/5) which I used to have (from new) in my flat, which was built around 2002.
The Viessman boiler may not have been available last year when I changed out my old Powermax unit, but even if it was, it was WAY too expensive as a replacement - costing over £4k for the boiler alone, compared to between £1k and £1350 for a standand combi boiler. Admittedly my home is a 2-bed flat with one bathroom.
What you have to make sure is the following:
Do you want a boiler/heating and hot water system that only provides heating OR hot water to taps at the same time. A combi boiler would be the way to go if yes, but for your home, I'd strongly recommend against that due to the high hot water load.
I only briefly looked at the Viessman units, but the 111 can produce 18L/min and the larger floor mounted version (looks much more like the powermaz and also has a far better heating turndown ratio than the wall mounted unit) - the 222 can produce 18-20L/min on relatively low gas input rates, due to the 'storage' design philosophy (my Powermax could produce 12L/min for a gas input of 14kW, the larger version 18L/min with I think 16kW or so, but with a 100L buffer tank instead of 80L).
Note that it appears that both the storage combi units cannot provide heating to the radiators AND hot water to the taps the same time (hot water takes priority). They are useful where you only have the space for that unit and not a boiler + (large) HW cylinder in the traditional type of installation, or for some reason have a hard to get to (i.e. to replace) AND small bore gas supply.
If you have a decent sized gas pipe which can take a higher load, and you have the space, I'd go for a traditional boiler + HW cylinder, though admitedly the cylinder will likely have to be around the 250-300L mark to cope with the three bathrooms and any other load.
It also depends whether you think that the odds of more than one shower/bath being used at the same time as another HW tap is quite likely, as 18L/min on a combi (35degC rise, i.e. around 40-45degC flow temp) is the equivalent to two showers in operation.
Any other outlet comes into use, sink, wash basin, hot water fed washing machine, the third shower, then the other showers will get a significantly reduced flow rate/pressure.
I'd also get a boiler and sysytem sized/configured to be able to heat the home AND re-heat the hot water cylinder (say 60 minute heat up from cold) simultaneously. That would be more expensive, but is very useful comfort-wise on winter mornings. Not doing so would be cheaper, but it means less heat available to quickly heat the house in the early morning should people want a shower.
One thing to note is that Viessmann, whilst a well-known commercial make, are far less known in the UK for domestic boilers, and far less plumbers are accredited installers and won't have quite the experience they would have installing/maintaining them as their 'primary' make - say Worcester Bosch, Vaillant, Potterton/Baxi, Ideal, etc. You may have to hunt around outside your town/area to find an accredited installer who you can get the longer warranty (10 years) from.
They won't be cheap either, including parts, but they are right at the top as regards reliabilibty.
Not sure about the heat pumps - never considered them before for domestic systems with a boiler already there. Note that heat pumps tend to use a lower flow and return temperature than 'traditional' boilers (pre mid 2000s, HTG on 82/71, afterwards boilers use either 80/60 or 75/55), e.g. 50/30 and require a large electric heating element to raise the hot water temperature to 60degC when required, but also need MUCH bigger (or deeper) radiators to get the desired output at the lower flow/return temperatures.
Such systems also don't provide a high output, and will need annual servicing from a competent provider (not a pumber) in A/C equipment. The outdoor units (if air source) can be difficult to site and rather noisy in comparison to a traditional boiler.
You may want to consider a solar thermal panel to supplement the heat input to a hot water cylinder (or storage combi if the tech allows for it - I don't know) if your hot water load is high all year around. More expensive (not sure if government grants are still available), and you may need to install an additional or set up a 'dump' radiator for any excess heat energy to go to in the summer when the hot water load is low.
Personally I'd just go with the boiler + large cylinder design, as many plumbers these days aren't so capable as they used to be - many are barely more than installers of bog standard equipment, rather like 'mechanics' at many car main dealers - they don't see enough 'unusual' work to get/keep the skills and knowledge to really know what they're doing installing and looking after them. You might get lucky though.
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