CS Cambridge/Imperial 3 A levels not enough?

I was stupid and didn't pick 4 A levels and instead opted to do just FM, Maths and Physics. This meant that I'd be technically sitting 2 A levels in y13 since in my college we sit Maths a year early if doing FM. I've seen others online with basically the exact same scenario get rejected from top unis and just realised I might be cooked.

Assuming I achieve 3A*, which I'm on track to do, and achieve a score high enough on the TMUA, how likely am I to get into Cambridge or Imperial? Will my PS have to be insanely good to just have a chance?

(I do qualify for contextual for Imperial if that matters)