connecting airport express to BOSE audio system

Are you NOT using Bose SoundTouch technologies? My experience is that is it more predictable than AirPort with Bose equipment. You also need to consider the roaming capabilities of your iPhone and Airplay and 3 different AP's (and the brand of AP). You may have to use the same SSID to allow the roaming to occur. Its an interesting solution, please post back your results. Thank you.

Mark

In addition to Mark Fitch's reply, your streaming network configuration should work. Mark mentioned a roaming network. This would imply that your AirPort Express and the three indicated Wireless Access Points would configured as per the following Apple Support article.

In practice, you will most likely find that iOS devices will tend to "hold on" to their connected Access Point and not roam between them as well as wireless laptop computers. This may or may not pose a problem in your scenario. Also, only a single device will be able to stream at a time. That is, you will not be able to stream from multiple devices simultaneously ... regardless if they are streaming the same original source material.