If you are within 21 days then you are able to submit an appeal to the crown court.if past that time then you are stuck with the 36 months. The ban should have been 23-28 months, with a reduction for the course if completed.
It puzzled me that you say you recollect them saying that you could get your licence back in ‘early 2023’….. even if they gave you the minimum 23 months, (which would be unlikely with a reading of 88) and you did the course, getting 23 weeks off, it would still be mid may before you would get your licence back, which is mid 22023, rather than early 2023.
The only way a court should give a 36 month ban for a reading of 88 is a second conviction in 10 years, or a serious accident with injuries to others.
If the court staff are amenable, it could be worth speaking to the court clerk, saying that you are mystified by 36 months for a reading of 88, point out that the guidelines are 23-28 months and that you are going to appeal to the Crown Court…. But it would save you AND the court a lot of hassle if he would look at the court file to see what was said about the reasons for the lengthy ban……..