Shelah the third son was reserved for the widow, yet with design that he should not marry so young as his brothers had done, lest he die also. Some think that Judah never intended to marry Shelah to Tamar, but unjustly suspected her to have been the death of her two former husbands, (whereas it was their own wickedness that slew them) and then sent her to her father's house, with a charge to remain a widow. If so, it was an inexcusable piece of prevarication; however Tamar acquiesced, and waited for the issue.
Related Commentaries of Genesis 38:11
Genesis 38:11
11 Then said Judah to Tamar his daughter in law, Remain a widow at thy father's house, till Shelah my son be grown: for he said, Lest peradventure he die also, as his brethren did. And Tamar went and dwelt in her father's house.