You can actually cut a yew, when ever you want throughout the year. But if you do it in the summer heat with the hedge trimmer, halfly cut needles will get brown. This does't harm the yew, it harms only the eye of the viewer.
Yew is extremely cut-resistant. You can cut off on many complete branches and trim a yew into any shape you like. You might also cut a spherical high strain out of it a squre, an Easter bunny or whatever. There are no limits, the yew tree forgives everything.
The only disappointment in the yew is only that it grows pretty slow. But in contrast to the 'so called' coniferes new shoots are also produced in the inner branches, as soon as some light comes into it. With such a radical cut it will last about two to three years until the yew is completely green and bushy again. Other conifers need 2 to 3 years until the first green tips, anyway. The Yew is our recommendation for a nearly perfect hedge plant. Even thought it grows slowly and is everything else but cheap.
The yew is a very remarkable plant, which may live up to1500 years or more! Moreover, it was the supplier for highly effective Germanic arrow poisons , but also tasty jams for food. A few branches of yew needles easily kill a horse, while the flesh of the fruit (not the core (!) is also poisonous, as all the rest ) is very tasty. Moreover, yew wood is the best bow timber and even the Neanderthals used it already for their spears. This was just a small dedication to the stunning yew tree ;-)
But a supplementary note. If your yews are "really" only three years old, they would hardly have 80 centimeter of heigth, and would be set with few side branches. Yours have probably been planted 3 years ago in your garden, but must be much older, right? If not, one rarely gets the idea to cut something that looks more like a skinny cloth hanger,
And by the way; No plant "must" be cut ! If your yew tree pleases you let it grow like it grows - if does'nt pleas you, you can do a section for a denser growth or a more beautiful form.