Tummy sleeves are great for third trimester! My mom got one for me, and I really like it for work. It's a one piece stretchy, cottony/spandexy tube that you wear around your belly, under your clothes. On me, it sits a little below my bra and slightly over my panty line. It gives very light support, and it keeps my outie belly button in, so my clothes look nicer. I also feel like I have more coverage, because now that I'm bigger, I can feel air touching my belly under my shirts, which makes me a little self conscious too. I do a lot of reaching at work, now I'm less worried about my belly hangin' out. It's not restrictive or tight at all, and so far I've never felt sweaty or gross in them either, they breathe very well.
Definitely recommend trying one out!
www.motherhood.com/Product.asp

This is my third and I feel like I started to show almost instantly. Everyone was guessing before I even told them. I think my body knows what's coming and so is just preping. Also, my belly has always been rounded so it's not like I was working from a flat stomach. EveryBODY is different though, I had friends who didn't show at all until their 5th month. But, it does seem that second and third pregnancies show earlier. As regards to belly-buttons, mine never popped out with either of my daughters.

well, i was all proud of my 18 week belly. there are pictures in my profile. but, I really popped out in my 5th month. though, by 4 months, some of my class was starting to wonder about the rounded belly; some were pretty sure and just didn't say anything until I did. :) I definately "had my nose" (as a dear friend put it ;) ) by Thanksgiving, which was... about 5 1/2 months? oh, pregnancy was really sweet- enjoy it! :)