Hi Alan and welcome. A diagnosis of CML is certainly a challenge - mine was an unwelcome Xmas present some 22 years ago now, but I am still here and doing well thanks to imatinib. Should you need any advice over the next weeks and months we are here to help.
To answer your questions: I am not sure about the 3rd vaccine/booster as I have only had 2 given that my S-antibodies are still at quite a high level (143) so my haematologist has agreed that I am not currently at extra risk. I am sure others will be able to advise on this and whether paracetamol etc. is helpful.
I suggest you read the various threads on Vit D3 levels, and If possible ask your clinical team to test your current D3 level. There is a lot of evidence that increasing your D3 level is very helpful to overall health and is protective against the worst effects of viral infections including Covid.
Once again, welcome to this forum,