3D Secure rules can be set to either one of these:
If the Rule is set to ALWAYS
Adyen will actively trigger 3DS on every transaction as long as the MID is enrolled for 3DS, the necessary information is passed in the API call (i.e browserInfo and userAgent) and the Issuer supports 3DS
If the Rule is set to PREFER NO
Adyen will not actively trigger 3DS on every transaction unless the Issuer mandates 3DS authentication for a successful authorization
You can set these up in the Customer Area > Risk > 3D Secure.