For me (a father), I would have to say I loved them (we had twins as our "second") at birth or soon after. Meaning I felt a strong desire to make sure they were well taken cared of and protected. Until birth they were very much abstract. But it was the same with our first.
They are 5 months old now, and I would have to say that the love for them as individual amazing little human beings is slowly growing. To be honest, I would have to say I still have a stronger love for my DD (our first) than I do for them.
I love them with all my heart. They are my children. But in some ways they are still kind of abstract personalities.
With my DD, I think I bonded with her quicker. Maybe because there was only one of her and I found it harder and more overwhelming with two infants. Maybe because I didn't have a three year old to also bond with at the same time.
I find that I was more hands on with her as an infant. And now my role is taking on more of the care of the older child to free my wife to have more ability to care for the babies.
But, you will love all your children. Your heart and sole has an infinate amount of love available.
It is hard to imagine that you can love anyone as much as you love your child.
Just before the twins were born I actually started to regret that they were coming and resent them a bit for interupting and changing the relationship we had with our DD. I felt a bit papa bear and angry at them for taking something away from her. I think that is pretty normal for parents to feel.
Having them has enriched all of our lives. My DD is loving having them to entertain.