So sometimes your device and WiFi connection sometimes can drop and if you haven't switched off mobile data, then the apps on your phone use your mobile data in the background. Apps are constantly using data in the background.
I had a friend on Vodafone pay as you go 1 tariff and as soon as you use data which apps do, it charged them a £1 because that is what the tariff states, £1 a day as soon as you use the service and you get some inclusive allowance for the rest of the day, so I advised them to switch off mobile data if you are not planning to use your Vodafone credit.
So back to you on Three pay as you go, if you have no active bundle or add on and your WiFi cuts off for even a minute the apps on your phone will use data and your cash credit will be used.
So like I said if you do not want a bundle and just want to pay standard rates which is about 35p per minute for calls and you don't want to be charged for data, simply switch off mobile data and just have your WiFi connection on. Remember to turn it back on if you are out and about and urgently need to use mobile data!
I hope this answers your question.
Also bare in mind if you are a very light user and do not use your account or credit for 6 months, you will lose your credit,phone number and account because all mobile networks disconnect lines not used for a long time, it's actually Ofcom who pushes that.
I recommend 1p mobile for low users, it uses the EE network and they have basic per minute charging 1p per minute for calls ect... they also have bundles for low users which can pay a yearly fee and lasts 365 days.
