Hi, the same happened for me. I went from a clear smear to CIN3 in 14 months and I had to delay my treatment by about a month because of visitors and it freaked me out that the cells were maybe growing at a faster rate. But everything turned out fine and remained CIN3. Everything indicates that they removed all the bad cells.
My friend had LLETZ done and after about 5 months got the all clear to start trying for a baby, but she ended up having CIN1. If your doctor confirms CIN3 from the LLETZ then she will most likely discourage you from trying for at least 6 months I would say as my doctor told me that it takes about 6 months for the cells to become mature again and you will be referred for a colposcopy rather than just a repeat smear at the 6 months to make sure nothing has reoccurred. If you fell pregnant before they gave you the all clear there could be a VERY TINY risk of developing an incompetent cervix. This means there is risk of premature birth because the cervix hasn’t had enough time to regenerate and ‘hold’ the baby up. Of course your doctor will be able to give you more guidance than me.
I know it sucks to have to go through this when you were just about to start such an exciting time of your life but like you mentioned it is good they caught it in time and will be able to treat it before you are pregnant and can avoid all that extra stress whilst pregnant. To answer your question in regards to if you are pregnant and the cells return that I am not 100% sure about but I found this link that has some information. http://www.cancerresearchuk.org/about-cancer/type/cervical-cancer/smears/pregnancy-and-abnormal-cervical-cells I am sure your doctor would address all these questions before giving you the all clear to try though.
Good luck with the treatment.