You will need plenty of fluid to climb a mountain and the best way to carry and access it is via a drinks bladder such as the Camel Bak brand. Most new rucksacks accommodate a drinks bladder in a pouch located in the main compartment with a hole in the top of the rucksack to feed the straw through. This will be closest to your back for stability as the bladder will be quite heavy when full. The straw will normally clip or attach to one of your shoulder straps so that you have easy access to it and don’t lose it round the back somewhere out of reach. This is the most convenient way of carrying your drink and saves you having to constantly ask a team member to pass you a bottle from your bag.
If you don’t have a rucksack that accommodates a drinks bladder you can buy a bladder in it’s own harness. This harness won’t hold many other items, so wouldn’t really be suitable on it’s own for the three peaks, but it may fit underneath your existing rucksack. Ideally it is worth investing in a new rucksack to keep weight and bulk to a minimum.
You can buy the Camel Bak drinks bladder on Amazon here.