Let's try to manually set your channels first. This will be dependant on other WiFi channels in your surrounding area, but this is a start.
In your Unleashed dashboard go to Access Points>Select AP1>Edit>Radio B/G/N (2.4G)>Override Channel: 1 / Radio A/N/AC (5G)>Override Channel: 157. Ok/Save.
AP2, follow the same procedure but for 2.4G choose channel 6 and for 5G choose channel 161.
AP3, follow the same procedure but for 2.4G choose channel 11 and for 5G choose channel 165.
Since you have no QoS in place and most of your users are doing simple web browsing, I would rate limit them to 1Mbps up/down. You can do this at the router level or WLAN level, and since we are on the topic of Unleashed: WLAN>Advanced Options>Access Control>Rate Limit>Uplink: 1Mbps Downlink: 1Mbps.
Each AP is on its own VLAN correct? What is the subnet size? When the network becomes unusable, are clients spread out across the 3 AP's or is one being overloaded?