In order to fully answer this question, context about 3D Secure in the world needs to be explained.
3D Secure is not unique to South Africa and is a system in use throughout the world as per Visa and Mastercard guidelines.
Visa and Mastercard divide the world up into regions and standards are implemented at different times in different regions as per Visa and Mastercard and local authorities.
Most issuing banks around the world have the infrastructure to process 3D Secure transactions and have been participants in the program for a number of years. This includes banks in all regions.
3D Secure in South Africa
As of February 2014, it was (and still is) required for all SA ecommerce merchants to implement 3D Secure on all credit card transactions. As a result, all SA banks are ready for it and will be used for all ecommerce transactions. Customers will be aware of it as it will be in use on all online stores and banks and merchants are together educating the card holding public about it.
3D Secure for international card holders
For international card holders where their territories have already undergone 3D Secure implementation (e.g. United Kingdom), there will be limited impact to them and they will mostly be aware of and familiar with 3D Secure authentication.
For international card holders where their territory has not yet undergone 3D Secure implementation (e.g. United States), there will be some educational challenges. For these cardholders, most of their banks are ready to process 3D Secure transactions, but because it hasn't been required by their local online stores, card holders are generally unaware of the program and little effort is spent educating the public about it.
This means that the 3D Secure page which appears from their bank, will be unexpected by a US card holder and they will more than likely not be aware of the program. They may also have challenges with regards to registration as their bank will not be used to fielding queries concerning 3D Secure. The US will be the last region in the world to see the introduction of 3D Secure, but the process is scheduled to start within the next few years.
The only approach that one can take to deal with this disconnect is to educate all buyers (but specifically foreign buyers) about 3D Secure, its benefits and what to expect from it. Educating them that it is a Visa & Mastercard program, that the 3D Secure page is served from their bank and if possible, finding resources specific to their bank to assist them in successfully using the service (these can generally be found online).