In theory, wireless clients attempt to connect to a wireless access point with the strongest signal. In reality, this is not always the case as you have experienced. Unfortunately there isn't much you can do to resolve this behaviour. It may be possible, due to Wi-Fi interference, that the signal strength varies just enough to not allow the client to connect to the physically closest base station.
As far as minimizing the amount of signal interference between the Extreme and the two Express base stations, you do have some control. What you cannot control, at least not easily, is interference caused by either the building material used between them and other Wi-Fis in the vicinity of yours.
What you can control is both placement of each of the base stations and what radio channel to operate them on. To help with placement, please check out the following AirPort User Tip. Some of the utilities mentioned in the tip can also be used to help determine which channel to operate on. You want to look for competing Wi-Fis. Specifically, the ones with the greatest signal strength and what channels they are operating on. Then change your wireless network to a channel that is at least 3-5 channels away. For example, if you find most operating on channels 6 & 11, change yours to 1.