I have clothes that are 3 months and I have clothes that are 3-6 months. Are they equivalent? is there equivalency between sizes that are given as a single number vs ones that are given as a range?
The X months sizing system for baby clothes is far from accurate. Cloths of the same supposed size (say, all 3 months) from different manufacturers or different styles can vary dramatically in actual size.
Then there's the fact that there is no standard size for a 3 month old baby. My 12 month old fits snugly in 18mo clothing. And my friend's 12mo old fits nicely in 9mo clothing.
The main difference between cloths that say 3mo and ones that say 3-6mo is that the company admits they don't know how old of a baby it will really fit, but think it's probably in that range.
From my personal experience, clothes that are X-Y are usually more towards the Y, meaning too big for X months.
We bought our daughter clothes for 1-2 years when she was year and half it for couple of months it was really loose on her - she has average build, not slender or anything. Now that she is 2 years old those clothes still fit, but pretty tight.
So unofficially I would say that X-Y clothes are bigger than X, although not sure what's the idea behind it.
The 3-6 months tend to be sized to fit up to 6 months, and the 3 months ones are sized roughly for 2-4 months. I would tend to treat the single age as the centre of a range.
For the clothes we bought (in the UK) I would treat these sizes as a very rough guide, though. Different vendors produce clothes of wildly differing sizes all described as the same so try clothes from the next size above and below and don't be afraid to get clothes that are too big, otherwise you will get to the end of the year and find stacks of clothes they never wore.