Yeah those two and PMC are the most science-y from what I can tell, that's why I went for them haha. Some Arts kid I am.
I think doing PMC in fourth year is a decent idea actually, that way you'd be doing it after you've done cog psych. It isn't a prereq for it or anything, but I reckon it'd be good to have a solid grounding in cog psych before you do PMC, just because of its reputation for being difficult.
I think that would most likely come down to personal preference tbh - like I said, RMHI should definitely be doable if you're willing to quickly catch up on ANOVA. That's the only thing you would have missed by not doing cog/social yet, so you should be fine if you choose to do it in third year. As for Psychological Science: Theory and Practice, that seems to be pretty straightforward actually, so I think you could definitely do that in third year as well. The only thing I'd warn you about is that you can't do Psychological Science: Theory and Practice until you've completed RMHI because the latter is a prereq for the former, so you won't be able to do PS:T&P this year and postpone RMHI til fourth. But yeah I think you'd be okay doing them in either third or fourth year, provided you put the work in haha.
You can find the subject guides for a lot of third year psych subjects (including both core subjects)
here, which might give you a better idea of what to expect and help you reach a decision. Careful though, cos the courses might have changed a bit since 2015/2016. I know the first four weeks of Neuroscience and the Mind have been removed and replaced with auditory neuroscience, for one.
Hope this helps a bit!