Agape is one of the least common words in classical Greek and it expresses the highest and noblest form of love which sees something infinitely precious in its object, in this case God. The heart here stands for the seat of all thoughts and emotions. To love the Lord our God with all our heart means to employ all our thoughts and emotions for that purpose. The soul here stands for the vital breath, the life element through which the body lives and feels. To love the Lord our God with all our soul is basically to love him with life itself.
Strength here stands for physical power.
To love the Lord our God with all our strength is to love Him to the point of physical sacrifice and suffering. And that leaves us with the mind . The use of the word mind here emphasizes our intellectual capacity, the thought process or the operation of reason. It is more than merely thinking of Him all the time. Our intellectual capacity is measured by our ability to learn and reason.