The reason for this is that the occupation of the two parts of the body in an act that Allah is pleased with is superior to the occupation of one part only. Likewise, the engagement of three parts is greater than two, thus the more the better.
Therefore, we should engage our three organs: the heart, the tongue, & the ears, when we are in Salah or busy in the remembrance of Allah.
By doing this, firstly your zikr will be counted as three times more & it will also be superior to the zikr with only one or two organs. Secondly, by engaging three organs at the same time it will terminate the evil satanic suggestions and thoughts that arise in Salah caused by the devil.
The method for this is that what ever you recite with your tongue; recite it with the presence of the heart, loud enough to hear it with your own ears. In this way, your three organs will be employed in the remembrance of Allah.
As well as this, if you count your zikr on your fingers rather than using a rosary, you will engage four parts/organs. This is also instigated in a Hadith, to count on your fingers. However, it is slightly difficult to engage four parts of the body in the remembrance of Allah at the same time.
(Only Allah can give the ability to act upon this, and Allah does not deprive his beloved dear servants.)
[Credit goes to 'Mountains Of Good Deeds In A Few Minutes' by Abu Talha Muhammad Younus.]