Hi - just doing 3 A levels and having a part time job is often too much! I would say no, you shouldn't try to do the equivalent of 6 A levels; A levels are usually a huge step up from GCSE and you may think that because you do 11 GCSEs you can do 6 A levels, but in reality, that's not a realistic path.
A level drama is quite practical with lots of coursework, and an essay based exam. You could do this instead of the diploma if you choose.
If you're thinking about how these qualifications may help you, take a look at what options are available to you post college/6th form. Taking the performing arts route, for example, you could study acting in University or at drama school, and these are usually VERY different training, perhaps similar to the difference between the diploma training and the drama A level training. Of course you don't need to decide for definite now so no need to over-stress! But looking ahead can make decisions a little clearer.
edit: if you want any info about the drama a level course, then I can help, just reply to this message.