Take no notice of the misadvice on here. The last thing you need is a cylinder/tank arrangement no more than you need gas light and a steam engine.
Explanation of an accumulator. They are super simple and passive devices. No pumps or other such garbage. They also store cold water too in a water outage.
http://www.heatweb.com/products/accumulators/accumulators.html
An integrated heat bank is what you need to supply three showers all at once - and an accumulator.
http://www.heatweb.com/products/cylinders/heatbank/heatbank.htm
http://www.range-cylinders.co.uk/products/flowmax
Get and "integrated" CH & DHW heat bank/thermal store. Run the CH off it and have the boiler only heating the store. Greater boiler longevity.
----------
Overview of Heat Banks
Instant high pressure hot water - When the thermal-store has reached the set temperature, water is delivered instantly at the taps.
Usable hot water in a matter of minutes - From a cold heatup usable hot water is available at the taps within minutes.
Very high water flow-rate - The high-end heat-banks have a flow-rate up to 45 litres/min.
Operates on low incoming pressures - Requires a supply pressure of 1 bar to obtain a good flow-rate.
Operates on high incoming pressures - Will operate up to 10 bar without a pressure reducer.
Long efficient boiler burn - Reduces boiler on-off cycling increasing efficiency. Inefficient boiler cycling is no longer a major problem with boilers with forced flues.
Maintains optimum boiler temperature range - The boiler Deta T is maintained giving boiler high efficiency and longevity.
Combines the output of the stored water and the boiler -
Cylinder may be smaller for a similar performance
Cylinder at low pressure - Unlike an unvented cylinder it does not store water at high pressures. No risk of an explosion.
Fast cylinder recovery rate - When the boiler is connected directly to the heat-bank or an indirect coil, the recovery rate is rapid.
Legionella bacteria eliminated - The Legionella bacteria cannot survive in the high temperature sealed conditions of a heat-bank's DHW section.
No scale build-up in heat-bank - Containing primary and not secondary fresh water, scale does not build-up inside the heat-bank, either on primary coils or immersion heaters.
Cold water storage eliminated - No need for cold water storage tanks.
Solar heating storage - Water heated via solar panels may be stored in the heat-bank via a solar coil.
Caters easily for multiple heat sources - Electric, gas, oil, solar, solid fuel and heat pumps can safely heat the one cylinder. The ideal solution.
Easy maintenance - If an external plate heat exchanger requires cleaning or replacing it is a matter of draining down the heat-bank, or closing isolating vales, and unscrewing the plate heat-exchanger. In some rare instances plate heat-exchangers are fitted directly inside the heat-bank preventing the unscrewing of the plate, however, de-scaling can be performed.
Easy to improve hot water flowrates - By simply adding additional plate heat-exchangers in parallel, hot water flow rates may be improved. Retrofit additions are possible if extra bathrooms or showers are installed.
Stored water vessel need not be cylindrical - As no internal coil is used for hot water heat transfer the stored water vessel may be any shape, as opposed to a thermal store which has to be cylindrical for maximum efficiency. Unvented cylinders need to be cylindrical to create a robust shape because of high pressures. This shape advantage is ideal where space is limited, promoting excellent packaging.
Integrated Heat Banks can run CH off cylinder - This enables TRVs on all rads when using an auto speed pump like a Wilo Smart or Grundfos Alpha. This give instant heat up in the mornings.
Creates a Superb CH buffer for free - A CH buffer is created, evening out heating distribution to demand. This puts less stress on the boiler.
Boiler sized for average not peak use -
Boiler can be a simple, cheaper more reliable boiler.
Boiler can be large and not cycle, giving fast recovery rates - Boilers can be large and be accommodated and not cycle as overlarge boilers do when connected directly to radiators. The heat up recovery rate is very fast.
Failsafe - When open vented, the system is failsafe and cannot explode.
No Overflow Pipes - On some models a discharge or overflow pipe is unnecessary. This means a heat bank can be fitted centrally in a house and not near an outside wall
Full DHW & CH Electric Backup - Electric immersions can provide CH & DHW backup.
