Three offers a basic PAYG Sim which doesn't require a credit check. It's 'traditional' PAYG (where a customer tops up using credit and uses that for calls, texts and data) and costs 10/minute, 10p/text and 5p/MB data.
The cheapest Three Sim costs £5/month and it currently offers unlimited minutes & texts and 2GB/month data over a 12-month contract. See cheap Three Sims.
Yes, you can keep your old mobile phone number. If you're moving from another network, you'll need to text 'PAC' for free to 65075 to request your PAC (porting authorisation code). You will need to provide this to Three which should move your number over within one working day.
If you are already a Three customer, you'll need to fill in a 'keep my number' form on its website.
Anyone joining Three after 1 October 2021 will pay £2/day to use their allowance within the EU. If you're travelling further afield, it's £5/day.
However, newbies to Three's PAYG Sim won't face these roaming charges, but you'll have to swap out your Sim before traveling, plus you'll have a different phone number. To do this, you can either top-up and pay its standard rates of 10p/min, 10p/text and 5p/Mb, or if you plan to use your phone a lot while abroad, it might be worth getting a bundle. Its cheapest is £10 and currently gives 10GB data and unlimited minutes and texts for 30 days. For more info, see Cheap Roaming.
Yes, Three offers 5G on all of its Sims. However, be aware you'll need a 5G-compatible phone too.