" Three " is the number of Divine perfection. This number shows Divine perfection within that which is in view. "Three measures of Meal"- three measures of ground grain, used to make bread- are in view. The reference is to the Word of God [Matt.4:4;cf.Isa.55:1,2],though not the word in general sense. Rather , the reference, contextually, is to the word in a specific sence, a specific part of the word, a specific teaching in the word.
The subject at hand has to do with the WORD OF THE KINGDOM. It has to do with how the message surrounding the coming kingdom of Christ would begin to be proclaimed in Christendom and how this message would progressively change because of something [a foreign substance] placed within the message [Matt.13:19-24,31,33].
And ,again, it is that part of this Divinely perfect revelation having to do with the Word of the Kingdom which is in view. Satan simply began placing those proclaming a false message about the kingdom among those bearing fruit for the kingdom. The false message took root and began to spread, resulting in corruption and deterioration. Understanding the parable of the leaven is that simple. This parable has to do with a progressive, continuing deterioration. It has to do with the corruption agent placed within that part of God's Divinely perfect revelation referred to as " the word of the kingdom." And it has to do with this corruption agent working " till the whole [the message surrounding the coming kingdom of Christ]" has been leavened. Near the end when the Word of the kingdom has been completly corrupted, that which Jesus fortold in his parable will be fulfilled. In those days, at that time, the true message surrounding the coming kingdom of Christ will not be- it cannot be-heard throughout the Churches of the land.