That's a bigger can of worms than you might expect. Most consumer WiFi chips only barely support AP mode, and I'm not aware of any that can do multiple bands simultaneously. You'd probably need 4 adapters on the repeater for triband. One to connect upstream, one for each downstream band. Three instances of hostapd all configured with the same SSID and auth for each downstream interface.

Then there's the roaming issue. This is largely what the commercial "mesh" systems try to solve: deciding / helping inform when clients should switch APs. There are many solutions and none of them are without issues, including the commercial ones. Here's a starting point: https://openwrt.org/docs/guide-user/network/wifi/roaming