If you're mixing all three, the pines will reach for the sky quickest, so I wouldn't plant them so they block the sun for the other 2 kinds. That said, it depends on how you're using the conifers. Screen - I'd plant the spruce every 12 to 15 ft. and then plant a second row staggered so trees in row 2 are centered in the gaps of row 1. If you're screening from a road or a neighbor ( again taking the sun into account ) I'd plant the spruce closest to the sun, next the cedars, then the pines furthest from the sunlight as they'll get the tallest - quickest.
If you're planting for winter thermal cover / bedding, I'd space them at 20 ft. if you're not going to later thin them. That way you get some space for grass & weeds to grow in between. Growth speed is still the same for all three, so you may want to think about your planting layout. Random plantings look natural and IMO make the best deer attraction for bedding and security cover. If you need thicker cover quicker, you could plant them at 8 to 10 ft. spacing and thin them later as needed.