What was the third servant thinking by hiding his talent in the ground?

Mt 25:

24 He [S3] also who had received the one talent came forward, saying, ‘Master, I knew you to be a hard man, reaping where you did not sow, and gathering where you scattered no seed,

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S3 thought his master was a hard man.

25 so I was afraid, and I went and hid your talent in the ground. Here, you have what is yours.’

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He was fearful. He was not thinking properly. His reasoning didn't make sense to the master.

26 But his master answered him, ‘You wicked and slothful servant! You knew that I reap where I have not sown and gather where I scattered no seed?

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The master would judge him according to his thinking of the master.

27 Then you ought to have invested my money with the bankers, and at my coming I should have received what was my own with interest.

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That's logical. S3 did not follow the proper logic.

28 So take the talent from him and give it to him who has the ten talents. 29 For to everyone who has will more be given, and he will have an abundance. But from the one who has not, even what he has will be taken away. 30 And cast the worthless servant into the outer darkness. In that place there will be weeping and gnashing of teeth.’

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S3 was an unproductive, worthless servant.

What was the third servant thinking by hiding his talent in the ground?

S3's thinking was flawed. His fear and negative impression of the master's character led to a failure to fulfill his responsibility. We are to actively use the gifts, talents, and resources God has given us.