One moment: the start of 1 January
Box 3 uses the value of your assets at 0:00 on 1 January of the tax year. For the 2026 return, that is the crypto you held when 2026 began.
Which value to use
The Belastingdienst says to use the value according to the price on the exchange platform you used. Its page does not say what to do if you used several platforms: note which price you used and keep a statement or screenshot.
Changes during the year
Buying, selling or swapping during the year does not change the tax for that year: only the 1 January value counts. What you hold on 1 January of the next year counts for the next year.
If you report your actual return instead, the changes during the year do matter: see the guide on actual return.
Every place counts
Coins on exchanges and coins in your own wallets all count. Moving coins between them changes nothing for box 3.
How Compass-Alpha shows it
The free tax estimate works out the value of your coins at the start of 1 January from your exchange files and shows the box 3 figure, with the savings, shares, ETFs and other investments and box 3 debts you type in for that year.