Source: direct literary origin of "three things a Buddha can not do"? Householder "?? English research institute" quoted:


  The three things a buddha can't do (如來 三不能 Buddha's three incapacities)
  1.不能免 定業衆生 He cannot destroy bad karma for anyone.
  2.不能度 無緣衆生 He cannot save the people unrelated to him.
  3.不能盡 衆生界 He cannot save the whole world of all living things


rightly answering here.

For the increase of faith and as possibility to make merits in such (as well as asked by ChrisW in a comment): 

Where can this statement be found literary in the basked of the heritage of the Buddhas teachings?

(Note that this is not asked for trade, exchange, stacks, entertainment and akusala deeds, but as a share of merits and continue such for release)
So far as I can tell these "three" are attributed to a Chan monk named Yuangui 元珪 (date 644 through 716).

An example story to illustrate  the first ("can't destroy karma") is his being unable to stop a king destroying the Sakyamuni clan.

I think the second means "can't save people who refuse to relate with him". The example story is of an old woman who disliked him when she saw him. He used his supernatural ability to miraculously show her everything, and she disliked that too. But she liked or had an affinity for Ananda when she saw Ananda, and so learned from Ananda instead.

The third is explained as something like, the people are (at least in part) saved or not saved through their own effort (their own understanding etc.).