Hi Dr. Stuart here, I'm a cat veterinarian in Ireland. It appears that none of my colleagues have been active online over the last few hours, so my apologies for the delay in responding. Thanks for your question, I'm really sorry about the three escapees, there is nothing so distressing as not knowing where a pet is or how they are coping without their usual source of food
Were the cats accustomed to close human contact? Had you been able to pet them when you were looking after them? Were they willing to be handled when moving to the new home or was the experience very distressing for them?
I would say that if they were reasonably confident around people then there is every chance they will return to the foster home. Although they were only there 5 days, it was probably still enough time for their new territory to imprint on them. As long as there was no undue stress in the home (for example excitable dogs or children) I would expect them to have remained somewhere in the vicinity. Having been street cats, they will naturally want to establish new territorial boundaries in their new neighborhood. They may therefore spend a few days wandering, but having grown used to being fed by humans, they will likely return to the foster home or at least venture into a neighbor that offers them food.
There are many stories of cats travelling great distances to their previous area of residence, and this is a possibility, but I think it more likely they will remain in their new neighborhood, especially given that there are three of them to establish themselves there.
Keep combing! If there are a few neighbours that are willing to leave food out for them it makes it all the more likely they will resurface before too long.
On a personal note, our beloved but highly-strung cat disappeared for 3 weeks within a week of us moving home about 10 years ago. We had gone through the whole grieving process when he arrived on our bed one morning bawling for food. In my half-awake state I honestly thought he must have risen from the dead! Wherever he came from, he's still with us all these years later.
I hope this is of some use to you, please let me know if I can be of any further assistance.