Answer: You can maintain the Dense Spreading Yew at a height of 3-5' and a width of 6-8' without stressing the plant. So, for a set of three, keep one at 5', one at 4' and one at 3' for the tiered effect you desire. You can prune them smaller if you're selective with what you remove and if you prune back as often as necessary to keep errant branches from emerging. Exactly how you accomplish this will really depend upon the growth habit of each individual yew. You may have to weigh the branches down to get them to grow horizontally, but it can be done. Bonsai or topiary employs similar methods of training and pruning. Perhaps a book on bonsai will give you some ideas about how you can train your yews. (Sometimes a picture is worth a thousand words!)
Best wishes with your project!
Q&A Library Searching Tips