when it comes to forex ads, there isn’t a single “best” country out of the three — results usually depend on testing. Finland and Denmark both have strong internet penetration and users are generally familiar with online financial products, while Belgium can sometimes be a bit stricter depending on ad compliance and targeting.
I’d suggest starting with small test budgets in each country and comparing CTR, leads, and overall cost. Once you have some data, it becomes much easier to decide which market is worth scaling for your specific offer.