Hi there. Ask away any time hun.
3.6 is below the average for 15wks, but is still well within normal range. A woman's cervix can be anywhere between 2.5 and 5 and be considered normal for them. Obviously the shorter a cervix is to start with, the less "room for manoeuvre" as the pregnancy progresses.
As your uterus gets heavier it will place more strain on the cervix, and it will probably shorten further, which is why they are checking you. They only get nervous if it drops below 2.5, at which point they may discuss placing a cervical stitch to keep it closed until it no longer is an issue after 26wks. At that point the baby moves up and onto the pelvis, so the cervix is under less pressure.
Resting can actually lengthen the cervix overnight - try sticking your feet up whenever you can. At the moment there is no need to panic - 3.6 is still a good length. I have always maintained that if every woman's cervix was scanned in pregnancy, there would be huge natural variation - and length would fluctuate from week to week.
It is definitely worth monitoring due to your previous surgeries. They will spot any worrying changes in time to do something about it. Cervical stitches placed before a cervix dilates are very effective hun. If next time you go, you have dropped below 2.8, begin to push for a stitch just as a precaution - that's if your doc doesn't recommend one anyway.
I had a suspected weak cervix in my second pregnancy, had a stitch placed in this one at 12wks, my cervix remained at 4 and closed til 25wks, and then funneled to the stitch and shortened to 2.8/3 from then on. Given that this is a twin pregnancy, it demonstrates even a really weak cervix can hold up if monitored and treated when necessary.
You may find that your cervix remains in the 3s throughout, and this can be perfectly normal, particularly if it started out shorter anyway due to your surgeries. It's starting point is also an important factor - it may have only been at 3.6 before you conceived, in which case it is holding up extremely well, and is normal for you.
Good luck - you'll be fine, and you are doing the right thing by keeping an eye on things x