Re: Three, Don't know where I'll go
Step 1. Search eBay for 'Classic O2 PAYG 321 SIM' you'll probably be able to pick up two for about 99p, the ones I recently received have an activate by date of 11-08-2021
Step 2. If you are looking to keep your number/s request a PAC by texting 'PAC' from your 3 enabled handset to 65075, you'll receive a text back from 3 asking you to include your date of birth, if you haven't registered with them any data of birth will do e.g., PAC010150.
Step 3. Once you get your PAC from Three, go to the O2 website and complete the Port-in request form, you will need to provide a name, email address, your new O2 SIM and Mobile Numbers, as well as the number you want to keep. Note that there is no drop-down menu for the Port-in date, so just write in the note section ASAP or a date you would like the transfer to happen.
As far as I'm aware O2 are still honouring the 3-2-1 PAYG rates for existing customers, but for obvious reasons you cannot get these SIMs direct from O2 themslves anymore ;-)
A bit like yourself I too have a number of devices and cannot justify bundle offers on all of them.