First of all, congrats!! I worked parttime in a preschool when I was pregnant, so I'm sure you'll be fine with #3, as you were with the 1st 2. Any baby is a blessing, so don't ever feel like you shouldn't have had one.
When I was pregnant with my daughter, who will be 6 months old on Thursday, I must have heard more unwelcomed comments than congratulations. I also got the "Wow, you're not due for another 3 months, you look huge!" This was especially welcome from men, as though they have any idea how I should be at that time. What can I say, my daughter was big, 8 lbs, 4oz and she was 2 weeks early.
My favorite comments, however, were more unusual. My husband and I were engaged since 2002, but had to postpone our wedding until 2004 because my mother was diagnosed with breast cancer. We had wanted to start our family soon for various reason, so our plan was to have our first baby in January or February of 2005. Well, she came about a year early, 14 weeks before our wedding. We didn't care as we were so excited to be blessed with a child, as we have several relatives who have been trying unsuccessfully for a long time so we knew what a miracle it was. When we told people I was pregnant we often heard "So are you still getting married?" Why wouldn't we!?!
Another great line we heard too frequently was "How are you ever going to finish graduate school now? You should just quit when the baby's born." To try and remain polite, which was difficult, I would thank people for their advice, but inform them I planned on finishing school. My MIL even asked my husband if he planned on dropping out of medical school! Why that would be a practical solution is beyond me.
In case you're wondering, we got married on June 5th and I was in class the day before my daughter was born. Those classes will be completed by this December and I'll be going back to finish my last year next May when she's 15 months.