All three have one thing in common: They represent three items with which Willy felt dominant, and which represent the power he no longer has. They are the power symbols of his past and the doom of his present.
The suitcase is the symbol of his job; the traveling salesman who, at one point, was successful (but now isn't).
The woman with no name represents how Willy is willing to ruin his marriage and be unfaithful to his family with someone who may have no importance in his life. Yet, for this very item of no importance, he basically ruins his relationship with his son.
The stockings, an expensive item back in the time of the story, again represents Willy as a powerful, lusty, businessman who can provide for a fling with expensive gifts (even if the woman will eventually disappear). If you notice, Willy continues to ramble about his wife's condition of her stockings, perhaps in a sense of guilt of what he did to her years ago, which was to give the other woman his wife's stockings.
In all, they are all fragments of Willy's past, and the symbols of his days of yore, who are now gone forever, and which he still mourns in bouts of insanity.
My interpretation of the symbols are:
The suitcases are symbolic of Willy's position which has required him to travel frequently. His life is wrapped up in the suitcases that take him from place to place further away from his family but never in reach of his dreams.
The woman is a sign of Willy's distance from his family and his need to maintain his self-esteem, and his loneliness. A salesman’s life is lonely. Willy was working to provide the American dream fro his family, but he was being unfaithful. In many ways this is a contradiction of what he is workings towards.
The stockings are symbolic of the gift he had promised one woman but given to another. If we look at Willy's life and dreams he is always lying to himself and his family. The stocking being given away is a betrayal of his promises to his wife.