How are you determining they're losing their wifi connection? Seeing them not available in kasa could be for a number of reasons, not just wifi. For the bulbs you'll need to look at your router's wifi status page - it's important to know what's happening for troubleshooting.

If it happens to all three at once you might try rebooting your router, if that fixes it for a while it's a router problem. If you have a lot of wifi clients it could be a wifi limit of the router.

You may be able to check signal strength for the bulbs by opening the kasa app. Touch the bulb name , then the "settings" button. Under advanced touch "Wi-Fi Network" and it will report the current signal status. Smaller than -50 is marginal, compare it with your plugs.