I'll try and break this whole paragraph down as I understand it, then we can tackle your question.
Russell is describing what you feel when a part of your body presses against something solid, such as a table. He says you feel that the table is "hard".
What he is then saying is that how the table "feels" to you differs depending on two other things: firstly which part of your body you press against it (for example your finger, your elbow etc); and secondly how hard you press (the amount of pressure exerted).
To summarise, he is saying that there are three different things contributing to what you "feel" - the table, your body, and the amount of pressure.
This is where it gets a bit philosophical. Russell appears to be saying that the "sensation" or "feeling" is not a single feeling, but multiple sensations. I suppose for example you have one feeling that your mind interprets as a measure of how hard, or solid the table is; you may also get a corresponding feeling in your body, perhaps a numbness or even a degree of pain from the pressure. There are possibly other feelings too. Russell then appears to be suggesting that this "collection" of feelings are not caused by any single one of the three things mentioned earlier, but by a "property" which you could not attach to any single one of them.
So to answer your question directly, I understand "them" to refer to the individual sensations which collectively contribute to your overall feeling, but none of which individually "contain" this collective "property" that causes them.
Reading The Problems of Philosophy beyond your quotation, Russell questions whether the table is real or not. So remember he is trying to get people to think abstractly, and separate physical object from the feelings you get from it.