How do I say goodbye to my father?
Robin,
This sounds excruciating for both of you. I'm so sorry.
I've had two friends lose parents to brain cancer.
While the 3 months nearly certainly won't be how long he lives (I hope for longer for all of you), I think it is, in one sense, freeing. There's no time for putting off talking about the BIG things.
I've seen family dynamics shift quite a lot once someone's gone and there's not too much to be done to prevent that. But, you can make sure that you feel resolved in your relationship with your father: say the unsaid things or forgive the grievances or clear up the misunderstandings.
Also, there are practical things that are great to get settled. Is this your role or someone else's? There are some useful resources here on caring.com to look through: 5 Necessary Legal Documents (http://www.caring.com/checklists/necessary-legal-documents-for-parent) and a support group families of people with cancer (http://www.caring.com/community/groups/cancer-support).
I know how much it must mean to your father that you'll be spending time with him.