honestly, tell them whenever you want. 3 months is good just because your first trimester. And be honest with them.
I started a new job on June 1, 2004 as a contractor. I found out on June 3rd I was pregnant. I waited 3 months to tell them because I was also waiting to get my first evaluation. I had to sleep in my car at lunchtime and I took along snacks and a drink to every meeting I was in. When they told me I was doing a great job, I told them, "I'm so glad you've been pleased with my performance because I just finished my first trimester and if you think I was doing a good job then, you should see me now that I have my energy back!!" They didn't even bat an eye at my news and I was hired perm when I was 8 months pregnant!!
Just tell them the truth.
Take your vacation if you want or modify it so you have more vacation when your baby is born. Either way, it's no biggie.
This is nothing to worry about. You're already a perm employee. I would be more worried about your maternity leave since you won't be eligible for FMLA. You'll only get the 6 or 8 weeks unless your company allows you to take more, which some do, some don't.