At GCSE my favourite subjects were Geography, Spanish, English lit and History (in that order) and my least favourite was RE. Because I go to a catholic school, I had to do an RE AS, which I thought I was going to hate.
Now my preferences are:
1- Geography 2-History (really enjoy these two, want to do geography at uni) 3-RE (unexpectedly enjoyed it this year, found it really interesting), 4) English lit (It's ok but really hard and I don't enjoy it as much as GCSE), 5) Spanish- at the end of GCSEs I was considering doing Joint honours in Geography and Spanish, but now I'm on the C/D borderline for AS...
Basically, if I'd taken 3 AS Levels I would have done Geography, English and Spanish, and at the minute I'd be on A A C/D, as apposed to A A A A C/D, while not fully enjoying two of my subjects (and a lot of geography unis are now asking for AAA/AAB). Therefore, I'd check to see if there are any subjects you find interesting (check the A-Level syllabus because some are quite different to GCSE, such as English language) or if any relate to extra-curricular activities you enjoy (such as Art, Drama, Music, Photography etc.), just in case it turns out you don't like or don't do as well as expected for one of your AS Levels (I got 98% for GCSE Spanish and in my first AS mock in January I got a D (one mark from an E), I didn't take to AS that well)
Based on History, you could try: Philosophy, RE/Ethics, MFL, Classics, Sociology, Politics or Psychology (I would recommend geography but I'm a bit biased!)