I'm just a student, but at one point I was experiencing being "not with it" in general that is very unusual for me.
I know people that always forget things, keys, phones, etc. when they visit and leave have to come back for them, or do things haphazard. I've never been that way, I instinctively do systems, keys in one or two places, mental checklist "do I have my phone, keys, house keys?" And when I organize or do things I tend to work out efficient ways to combine tasks...normally. But there was a period of time where I was just not able to concentrate well. It passed, but in that time (and we all have "those days" from time to time...when I get my cert. I definitely won't fly on days I feel like I'm not with it) I felt really slow, and distracted.
So I'm wondering, other than when flying, the three incidents you had, have you been also noticing being out of it and distracted in everyday life?
That splits up the diagnoses. Either it is a general malaise, a period where you are not up to par, in general health, OR it is only noticeable when flying.
If it is only when flying, I thought the same as some others here, can it be that since you have been flying so much lately you are getting complacent? I really don't understand why you aren't using actual checklists.
Again, student here with not much practical experience, but at our school we have a laminated little book of checklist and emergency procedures. We are required to go through the checklists. I don't know how others do it, but other than the standard inspection, before taxi, run up, before takeoff, we also have "before airwork" and several others, and before landing approach as well. I believe it would have uncovered every one of the three mistakes you had. It seems you are getting a kind of blind spot (if this isn't general, only flying) about procedures?
If it were me, I think I would take a little time off of flying, try again in a week or two and see if it helped.