Combined is all 3 - AQA call it Trilogy rather than Combined.
6 papers, 2 in each science, marks all added together to give a total number and that number goes against a scale that allocates you 2 GCSE marks - so a 7/7 or a 5/4 or whatever.
You can take Higher level or Foundation level - Foundation being max of 5/5
Triple is three separate sciences, with separate grades based only on the 2 papers in that science, not a combo.
Syllabus for Triple has more content.
They should, imo, offer the option to just sit one of the Triples as a Science option, or come up with some new Science for non Scientists worth 1 GCSE for those who would prefer to spend their option elsewhere.
We just had to pick one Science when I did GCSEs. I have found the 3 languages I was able to take vastly more useful in life. My lack of Chemistry qualification has never felt like an issue.