Man vachan kaya are the three fundamental principles used to define non-violence (ahimsa) and truthfullness (satya). Being truthful means whatever you are thinking, speaking and doing, it is the same.
You should be one in whatever you think, do or speak. Ahimsa is the fundamental principle of Jainism which means you should be non-violent not only in your actions but also in your speech and thoughts. Being truthful makes you Satyugi.
Only in certain circumstances, you may differ in your thinking, speech and actions. For example, you cannot tell a patient he is going to die in the next week. Here your mind may be saying something, your speech and actions will be something else.
Any discordance in Man Vachan and Kaya ends up with guilt and suppress emotions which later on can cause blood pressure, diabetes and heart disease.