You can bind on the event “OnClicked” of each button.
I will probably do it in this way:
When (for example) the left button is clicked, you can start a timer that will increase a float value to a max or to a value you want.
In BP you will do: “OnClicked”->SetTimerByFunction (or "SetTimerByFunctionName).
This function will have the duty to increase your rotation angle till the button is pressed.
When the button is released, you will “ClearAndInvalidateTimerByHandle” of the function so that your rotation angle will stop increasing.