you might need to forget them on the app you are using but thats probarbly happened already as it will not be able to find your old wifi - so you need to set up the app to the new wifi on the hub3 - you also need to reset the smart plugs and re pair them - usual method is hold the button on the smart plug for 5 or so seconds until you get a quick flashing light
smart plugs work on the 2.4 band usually - so set the app up to the 2.4 band but before you do that split the bands on the hub3
if the above does not work it means the wifi from the hub3 is not good enough for the position of the smart plugs but thats another problem - see how the re setup works and come back if you still have connection problems
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Tony