Stop worrying. You're still in shock about the pregnancy - that happens to so many of us who have dealt with infertility. We think we'll have kids with no problem. Then we face infertility realize how "off" we were. Then we think we'll never get pregnant, or never get pregnant again. Then an unexpected pregnancy occurs, and we're "off" again. It affects our view of ourselves, and then it permeates our view of our family life and our confidence and many other things.
Kids adapt to whatever reality they are presented with, if the parents are calm and confident. Siblings are not the same as friends. My stepkids were 16 and 13 when our son was born - far too big a difference to be playmates, but they had a relationship based on this age difference. My neighbors have 3 kids exactly 2 years apart, and others have kids 4, 7 and 12, and another family has kids 7, 9 and 13. There's no magic formula for getting along and having things in common.
Your 2 year old is not going to have any more in common with the new baby than the oldest is. You'll find different activities, or you'll do things like museums with something for everyone. The oldest can help with the middle one in the kiddie section, and the baby can be in a Snugli-type carrier. The middle one can just look at the animals in the zoo while the older one reads the descriptions and discusses it with you. Everyone can go to the beach - the oldest one swims, the middle one stays in the shallow area, the baby naps in a beach tent. The older two build a sand castle with the older one designing more and doing the detail work, and the middle child doing the moat and the trenches. Little kids admire the older's expertise, older ones get to experience the pleasure of little kids' toys (things they are too "cool" to play with except in the context of entertaining the little sibling).
Big one pushes the stroller, middle one stands on the back, baby sleeps. Over time, the oldest will be able to help you watch the baby (sometimes as a good family member, sometimes for pay). The oldest will get rewards in being the first one old enough to sit in the front seat of the car or get a learner's permit. The oldest sometimes has to adjust to younger kids, but also always gets new stuff and never hand-me-downs - it's okay to point that out if she gets snooty at some point.
Sometimes you just get a babysitter, or split up Mom and Dad. That's okay. You get to have special time with each child and recognize their differences.
It all works out. There is no perfect set-up where no one gets their nose out of joint and there's no arguing. There's no predetermined age breakdown where they all get along perfectly either.
There was room in your family for Baby #1 - you and your husband adjusted and, no matter how wanted that child was, your relationship changed. Somehow, you managed to incorporate Child #2, and everyone adjusted. The "only child" maybe loved it, maybe didn't, but it worked out and you now have a "family dynamic" that seems perfect. You'll adjust again with Baby #3 who, like all kids, brings new joys and new frustrations and a new personality. All 3 kids will find friends their own age and many fun activities, just as you and their father enjoy family time but also your own adult friends.
Learning to adjust as a child and make room for different people (ages, personalities, needs) is a great skill for learning to socialize in the teen years and the college years and the adult years. Kids are resilient. You are not throwing them for a loop by having a 3rd baby - you are building family and memories and abilities.