Universities usually expect you to have 3 A-levels when applying to degree programmes, and I think taking 3 A-level in a single year is going to be very challenging. Even taking 2 is quite likely to be a big challenge for that matter, particularly those subjects you've chosen - also bear in mind universities usually expect applicants to STEM degree programmes who are taking science A-levels to sit the science endorsement (practicals). For 2 sciences, doing all these in a single year as well will also increase your workload.
I would recommend just doing 3 subjects over 2 years, as is typical. This will give you the best opportunity to do well in them, and to fully understand the material (which will be important if you go on to a degree programme using those subjects). Your experience of starting uni at 20 vs 19 isn't going to be really different (or any different from starting at 18, for that matter!) so the extra "time" taken isn't really a problem.
If you really feel you must do a course in just one year to go on to university study, I'd probably suggest instead looking at Access to HE courses; these are designed to be taken intensively in a single year, and often include more study skills support for students on the course in view of this. The only caveat is that not all Access to HE courses are accepted by all universities for all degree programmes, so you will need to contact those you may wish to potentially apply to and check if the specific Access to HE course(s) you're looking at are acceptable.