Hi. I have a three and a half year old and one year old twins - all boys. I was also afraid of my oldest feeling left out. I've read that the sibling of twins might start asking about how Daddy has Mommy and the babies have each other, but who does he have? While he did mention the other day that it's not fair that he has to sleep alone (oh dear, it's starting!), I'm really going to downplay it, as I am definitely NOT going to have anymore children! I love all my boys, but a fourth might just send me over the edge! :-)
I think it will be interesting to see how the dynamics play out later on. Both boys love being with their older brother at the moment and their older brother likes to find a place to play quietly where the babies can't get his toys. It's great, because he was never very good at independent play, so this has taught him to play alone, oddly. It might be that or an age thing, who knows? In any case, I think it's good that the genders of your children are different. When I was pregnant with the twins I thought a lot about our oldest being the odd man out and hoped that the twins would either be two girls or one of each to better the dynamics between the children. Didn't happen, but now we're here and we will work hard to make sure our oldest doesn't feel left out for sure. And in fact, it's not nearly as bad as I thought because I did ask him if he wanted to sleep with his brothers or in his own room at Grandma and Grandpa's house and even though he knew his brothers would be sleeping in with his daddy and me, he still chose to be on his own.
To address your concerns about having three under 4, well, I'm in the thick of it. It isn't easy, but there are some advantages in that you get it all done and over with quickly! I know people who have waited five or more years between children and I can honestly say that I wouldn't want to do it that way. Once you leave the diapers/sleep deprivation stage, I think it's very difficult to go back. My challenges in having three this young, though, are definitely taking them anywhere on my own. It was fine when the babies were content to be in their stroller, but now that they want to be out and crawling around, well, it's gotten more difficult. For instance, I took them all to the park one day and unloaded everything, got a picnic blanket out, put the babies down on it and got some food out, etc., when my oldest ran over from the slide/swing area to inform me that he had to pee ... seconds after I'd just finished unloading everything! So I had to load the stroller all over again, kids and all, and take him to the toilets. He did the same thing leaving (he insists he doesn't have to go when I ask him) and fortunately we were parked beside some trees... I think that when the twins start walking, it might get harder again. I know I can use harnesses, but I'm not sure it's going to work out and if they're like my first (runners), then I won't be able to run in three directions at once, which greatly concerns me. Thus, look into hiring a babysitter if possible if you're going to have a third!
Breastfeeding the twins was also a challenge as if I was by myself, my oldest would look on and get so jealous of their time with me that he would proceed to draw on the walls, take things and make messes, etc., right in front of me and because I was tandem feeding, there was not a lot I could do. A box of toys for nursing time only was suggested, but nothing like that worked for me. I weaned them during the day, then, and only breastfed at night after they were three months old.
As for your age, you don't say how old you are, but I do find that people in the US tend to have children way earlier than we in Canada do. I was 35 when I got married and 41 when I had the twins. Sure I would have liked to have children earlier, but it didn't work out that way. So, if you're in your early 30s, I wouldn't worry about age. There are risks, yes, but honestly, even doctors are finding that women are more able for childbirth at an older age these days. So when they say 40 is the new 30, they mean it! ;-)
A wise friend once told me that you can never be ready for children and it's true ... whether it be your first or your fourth you can always make excuses for putting it off or you can dive right in head first. It just depends on what you really want in the end. Listen to your heart and envision what kind of family you see in the future... two, three, four.... Hope my ramblings help!