It seems to vary from hospital to hopsital whether they actually tell you about margins, although with CIN3 they would call you back sooner than 6 months if the results from biopsying the sample showed that there were abnormal cells on the edges (margins). So you're probably safe to assume that the margins are clear, although you can always speak to your GP or the clinic if if would help you to be told the margins are clear.
The 6 month check up is something everyone has after treatment. Since introducing HPV testing, that's part of it now, it's called 'test of cure'. So if your smear is clear and you're HPV negative, you'll go back on to normal 3 yearly screening. This is because studies have shown that you would be likely to be 'cured' (as most women are after LLETZ).
Hope this helps, all the best to you.