Can you show a picture of the situation? And say which direction the sun comes from? You’d need a huge amount of space for three rows of planting. And I don’t quite understand why you’d plant the Snowballs ’behind’ the conifers. Wouldn’t they be hidden? And I wouldn't plant them between two rows of light blocking conifers.

You location would be helpful to perhaps suggest alternatives. Soil condition matters as well as rainfall. "Snowball" implies viburnum and there are many beautiful varieties. More information, like dimensions of the planting area, sun, photos would be helpful.