It makes sense to visit countries close in proximity, relatively small in size and similar in culture all in one go. Here are some destination suggestions that give you bang for your buck(s). One plane ticket is all it takes to see not one, not two but three countries!
Jordan, Syria, Lebanon
Cambodia, Vietnam, Laos
Mexico, Guatemala, Belize
England, Scotland, Wales
Any 3 adjacent countries in Europe
Of course there is much to see in each country for each trio. And while the cultures appear similar on the surface they are still quite distinct. However, if you are on a budget and want to reduce the amount of hours of flying time and jet lag you have to experience in your lifetime then it makes sense to see as much as possible when convenience and finances allow.