How do you feel if this happens?

Say you buy three of the same plant. Say a heather. Things always look better in 3s, the effect is stronger.

But one dies. So you're left with two.

If you cant remember the exact variety/cultivar, would you use something really similar? Does it really matter that much?

I'm trying to decide if just having two would look worse than having one with a very very slight difference, next to two others.

 

As long as it’s the same or a similar colour I wouldn’t worry. I’m often replacing or relocating Nepetas and forget which varieties they are.

 

Wouldn't bother me, plants are plants.

 

Most of our garden plants can be fairly easily propagated so I always take cuttings/ divide clumps just in case.

To be honest, that's not always the best policy as I'm unable to get rid of the surplus so often end up with not a neat threesome but with a thirteensome

Obviously, the propagated one will take time to reach the same size as the two original members of the threesome but with permanent plants that's not a big issue.

 

I don't worry about the 3, 5, or 7 rule. When I first started gardening, I would buy 3 of something and then dismay would set in as I found they didn't thrive in the spot I had chosen and needed a new home, but where to fit them all? Or, as you say, one would die. Or occasionally I'd find one had been mislabelled and was not the same at all. Or they would seed around and I would end up with them all over.

So, I almost always buy single plants and either let them grow into a bulky mass or propagate them if they are a huge success.

 

I agree that things look better in odd numbers but unless you've got a big garden you don't need three. One is an odd number and I think that's enough.
Maybe if you've got two you could plant them separately from each other, then they would give a better look, like 'one' and then another 'one' ...