How can I introduce two new cats to my one indoor cat successfully?

You can get them used to each other's scent by rubbing a cloth on the established cat and on the new cats, the theory is that if you do this for several days, the new animals will smell slightly of the established cat's environment. I have done it by keeping cats in separate areas, so that they can see each other and smell each other, but not touch for several days. Each cat will need their own food bowl and litter tray, preferably out of sight of the others, or they may start marking territories by spraying or soiling, which is unpleasant to deal with.