It's getting harder and harder to find good redwood with vertical grain clear heart. Personally, I've never used 3" thick stock. Genererally, we use 6/4" and occasionally 8/4". You're basically looking at 4" x 4" posts, which I highly doubt will be any good for gluing up to your required size. The grain will probably make it twist over a short time. Your widest plank will be about 11-1/2" wide, so it will take eleven pieces 7' glued on top of each other. However, if you're asking these basic questions.... you might wanna align yourself with someone locally that can do this for you properly.
3" inch verges on lam beams... not really considered sandblasting substrates.