In Toronto. Zone 5 – 6. I have three Japanese yews, pyramid shaped planted to be a barrier from public scrutiny. Two are flourishing. Replaced the third twice. The needles yellow and the plant slowly dies. The soil is basically clay and is much the same for each plant, as far as I can determine.
The Japanese Yew (Taxus cuspidata) is commonly used as hedging, for screening and foundation plantings in a large landscape. They are considered to be slow growers, so you will have to be patient if you want them to create a solid barrier in a short space of time. They can grow up to 15 metres and have a spread of up to 8 metres and that is what makes them effective barriers. Their native habitat is N.E. China and Japan so they will tolerate growing in Hardiness Zones 4-7. Toronto fits the bill nicely.
Yews grow equally well in sun or shade but they hate windy sites. For a Toronto winter its a good idea to protect them from the wind with burlap or a similar product. Yews do appreciate good soil that is well drained. “Nothing kills a yew faster than wet, poorly drained soil”. The yellowing leaves on your yew seems to indicate a wetness problem. Since you have two yews that seem to be doing well the drainage is probably okay around them. It could be that water is pooling around the trunk of the sickly yew. If your soil is predominantly clay it will have poor drainage, so it may need amending. Try adding compost or triple mix to the soil around your yews. This should improve the drainage.
The good thing is that yews are resistant to most diseases but can be attacked by caterpillars, weevil, scale and mealybugs. From your description its doubtful insects are the problem.