Kelly and I talked to a lot of fosters before we took Ripley into our house. Our original goal was to find her a good and solid home, that didn't work out the way we planned in the end, but we went into it with the same concern you have.
The feedback we tended to get was simply:
"It Depends on the dog".
Some dogs deal well this the separation and are fine with new pets coming in and out as they go through your fostering to their forever home.
Others however develop a strong connection with a Foster and can get attached. When the foster leaves it can be hard on the pup at times and they could become lethargic for a period of time before they "Move on" so to speak.
In the end dogs are Pack animals. They like being together, for the most part. The longer a pack is formed the harder it CAN BECOME for some dogs to let go of their pack.
Routine, exercise, motivation, and excitement can help in this regard. Filling the void left by just being there for your pup and doing things together goes a long way in getting over that initial phase. Once some time has passed, and if you don't dwell on it, most dogs won't either. As long as your personal pack is solid your pup will come back around, after all it is common place for packs to split all the time in the wild for a number of reasons.
They truly do tend to live for the here and now, and as long as you yourself aren't saddened by the re-homing of a foster your pup will get over it in due time as well... Just take the time to really be there.
Like anything the first time is the hardest, on both of you, and it gets easier as you both get experience.
That being said almost all the fosters we spoke to about the subject agreed on one thing; It tends to be easier if you have 2 dogs and are fostering the 3rd. Most of the people we talked to started off by Fostering their 2nd dog and keeping it. They were able to part with the rest of their fosters after the first..... Something Kelly and I are Kinda familiar with ourselves.....
Either way these are just our experiences and those of whom we talked to. I'm sure others have had some different experiences they could and will share
For instance Tori's dogs apparently didn't seem to notice Ripley was gone
If you do decide to foster GOOD ON YOU!
Is this about the cake problem? What's the matter with you mathematicians, cake is never a problem. - Professor Lazlo