My apologies - everyone, I have given you some information that is not quite right above (too busy trying to post here and do my normal job).
The 3rd conditional is fine. However, the part that is incorrect is the second conditional - you can't use the second conditional to expand the information here and well done to anyone who has spotted my mistake - top marks!
What you need instead is a mixed conditional:
Mummy shouldn't have drank that coffee. If she hadn't drank the coffee, she would be able to sleep (later tonight). (However, she did drink the coffee, so she won't be able to sleep.)
I'm making more mistakes than anyone today and I'm supposed to be the native speaker!
Anyone who has spotted I used V2 above instead of V3, go to the top of the class.
The sentence should read:
Mummy shouldn't have drunk that coffee. If she hadn't drunk the coffee, she would be able to sleep.
Final Answer! Arrrrghhhh. This English thing is sooooooooo hard!!!!!