The Gateway included in ActiveMerchant does not support 3D Secure. This implementation was done by David Rice and sponsored by Ticketsolve.
In your Gemfile
gem 'activemerchant-realex3ds'
In your code
require 'active_merchant'
require 'active_merchant/billing/gateways/realex3ds'
gateway = ActiveMerchant::Billing::Realex3dsGateway.new(
:login => 'TestMerchant',
:password => 'password')
See ActiveMerchant for more details.
Ported to current version of ActiveMerchant and extracted into a Gem by Arne Brasseur.