The wifi roaming is a function of the client device.
So to encourage roaming, you will need to turn down the tx power level on each access point.
Some windows laptops allow you to access the wifi device hardware settings and there is sometimes a roaming stickyness slider where you can make the laptop super sticky, or more willing to roam. The cost of roaming more is the higher chance of downtime as the wifi card changes access point.
So by lowering the power level on the 'master' access point, a client device will think its getting a better signal from one of the slaves, and decide to roam sooner. This is helpful when devices such as ipads, generic windows wifi drivers or other simple devices dont allow you to choose the roaming stickyness.