A level crisis!!!!!!!!!!!!

So I'm in year 12 and currently do 3 A levels which are biology,chemistry and betec applied science level 3 currently I'm absolutely f*****ng failing A level chemistry which is unfortunate I stuck however in the event of what to do if I fail A level chemistry and walk away with 2 A levels like wtf am I supposed to do my main question is tho can I do a additional betec in a year after year 13 and apply to uni the following year with enough UCAS points however I have received mixed messages with some telling me unis won't accept qualifications taken in different SITTING plzzzzzz tell me more about the sitting idea and if there is any substance to it