Your 3 APs should be configured with identical SSID information. They will advertise and service your SSID all the time. The APs do not sleep.
Are you sure that your three APs coverage overlaps, so customers are always able to see at least 1 AP advertising your SSID? You can do a simple site survey as you walk from one side to the other. Stop as you walk, and scan the air for SSIDs seen. Do you see your SSID advertised by at least two APs in each location that you stop?
Lastly, do all customers report the problem or only users with one type of client? It might be the client that is going to sleep or having issues roaming. Because you use three different APs, your clients will be doing a new DHCP request with each roam. If using 802.1x SSID, they will be re-authenticating with your RADIUS server with every roam, before doing new DHCP.
I still don't know all your variables. Only ZD controller management of the APs would make roaming more fluid (cached credentials, 802.11k/r).