My wife and I had 8 kids and from my experience the closer together the kids are the better they play together, and do things together as they are growing up.
Also, my job put me in contact with a wide variety of middle class men and women. The subject of children often came up. What I learned is four children appears to be the magic number of kids for a family to have. If they had one child, they usually wished they had had three more (makes 4). If they had had two kids, they usually wished they had had 2 more (makes 4). For those that had 5 or more kids they were usually happy with the number of kids they had. The only exception to this was the families tha had a child go on to "institutions of higher learning" like Alcatraz or San Quenton. Then they usually wished they hadn't had any kids.
As far as your kids being friends, it depends on how you raise them. My parents were both only children. They both told me the reason they had two kids was they both hated being only children. But they didn't know how to raise two kids to be friends and so my brother and I grew up to be enemies. I studied to get B's and C's. My brother could sleep through class and get A's. When report cards came out my dad would gripe at me and ask why I couldn't get A's like my brother. When I was invited to go someplace, like a birthday party or to go to the zoo with friends or neighbors, my parents would tell me that I couldn't go unless my brother could go too. This was even if the neighbor only had room for one in their car. I gave up even asking if I could go anywhere without my brother. My dad and brother would tease and tease and tease.
I didn't allow any teasing in my home. I never compared one child with another. We did praise for all my children. We rejoiced in one child's C's and another child's straight A's. It all depended on their abilities. I had one child that had a hard time getting B's. I found out he had a hard time and was getting D's and F's because he wasn't reading the books and wasn't doing the home work.
All my kids are close and are friends. The oldest is 14 years older than the youngest and they are not as close, but they are friends. All my kids with kids delight in havng their kids play with their cousins.
If I was in your position, knowing what I know, I'd have the fourth child. I've always looked upon my children as assets and not liabilities. Now that I have grand children, I can't imagine how much poorer my life would be if I'd only had three kids.
Good luck to you and yours.