This "rule" about odd numbers of plants was conceived as an easy way to help the less experienced achieve a better look. But what really matters is how things look, not the plant count. It just happens that in some cases, odd numbers of plants look better. But not always. Imagine a bud vase with a single stem rose in it. Looks pretty good ...? Add another rose. Is the looks as classy, or does it look a little dorky? Add a third rose. Now, it looks better again ... more full, rounded ... view-able from any direction. Add a 4th. Seems a little strange. Add a 5th. Now it's starting to look like a respectable bouquet ... a little skimpy but not unpleasant. Add more roses and no one will be counting them. It just looks better the fatter and fuller it gets. In general when you add upright plants in the landscape, it looks better in the odd numbers, but after it becomes a sizable group, then no one counts and no one cares.

Usually, in landscaping, one is creating large form & shape "statements" using several or many individual plants. The number of plants used would correspond to the shape and form being made and the space allotted. In the non-upright plants (such as shrubs and perennials,) the "rule" of odds doesn't really apply if the plants are going to grow together because then the observer would not have any way of knowing how many plants went into making that particular form/shape. If one is going to maintain each shrub or perennial as an isolated individual, well then, the end result is not likely to look good anyway so they shouldn't worry about whether the number is odd or even. That's probably a little harsh and I'm sure there are exceptions, but in general it would be usually more true than not...so about as good as the general of using odd number of plants. In the case of your example, one would need to see the space to determine if the arrangement you're making fits and suits the space. And if the plants you've selected are appropriate, in order to know if it's going to look good. If you have a tree centered between some shrubs, what's the total length of the arrangement and the total length of the space allotted? How does everything work together? That would more likely determine whether you use two or three shrubs at each end of the tree. I can't see what's in your mind, but I would most likely be visualizing a tree centered between short hedges as the shrubs would be growing together. So that's a tree with a small hedge on each side. Three things.

What comes to mind as the notable exception to the odd/even "rule" is when one is creating a gateway ... an attempt to "formalize" a particular place where one would pass through a space (either visually or physically.) This could be done with a single plant on each side, 2, or multiple plants. What matters is that each side match the other.

Wow. That's more of an answer than I as expecting, but thanks for such a great answer.

So it's not just a matter of Odd Groups but also having to keep the concept of Balance in mind as well.

In my completely fictional example (seriously, I don't have a situation like this in my yard yet), it would also sort of work to have a single larger viburnum, that is roughly double the size of the dwarf hollies on one side of the ornamental and two dwarf hollies on the other side. I imagine this would have to be very location dependent, like sitting near a inner-corner where the viburnum wouldn't stand out as being out of place. It might even work to have three hollies on one side and the viburnum and another holly on the other side, so long as the weight is the same on both sides. Again, it would have to be in a particular area that would support this... maybe on a back corner of a house where you want the viburnum to sort of block what is around the corner but don't want the hollies blocking the trunk on the ornamental as you approach the corner.

Thank you for your very descriptive and thoughtful answer. It gives me one more thing to keep in mind as I figure out how I want things to go.

"It might even work to have three hollies on one side and the viburnum and another holly on the other side, so long as the weight is the same on both sides. " It's not the weight of the plants you're immediately looking at/working with. It's the weight/balance of the entire composition. One must take in to account what's happening fore and aft, left and right of the immediate concern. You might have a tree and then all shrubs on one side. It depends on the setting. In a foundation planting, for example, architectural features such as door, windows & other architectural details often (really often!) determine placement of plant material that goes in front of them. There are innumerable variables, but overall balance coupled with visual interest is the main thing to strive for in the finished product. (In addition to specific site and client goals.) As you move about and observe existing landscapes you might pay attention to small groups of individual items, check their numbers and get a feel for how successful or disturbing those numbers are to you. I speculate that groups of 2s & 4s might look "off" unless they're forming a gateway, and that groups beyond 5 you'll think the count seems unimportant unless it's a widespread group.