There are two options when it comes to multi-platform use of Valentina Studio Pro.
Buy one of each OS version. Valentina Studio Pro is platform specific, so if you go this way, you need to get one of each.
Valentina Studio Pro Universal. This is a single license, but it works on macOS, Windows and Linux. It is a 'per user' license, so it assumes you are using only one of them at a time.
The cost of Universal is almost the same as getting two copies of Valentina Studio Pro, but it works on macOS, Windows and Linux. Three for the price of two is a good deal when the difference is so small.